diff --git a/pom.xml b/pom.xml
index d036d0d..59bb365 100644
--- a/pom.xml
+++ b/pom.xml
@@ -28,7 +28,7 @@
 		<dependency>
 			<groupId>com.github.informatik-ag-ngl</groupId>
 			<artifactId>envoy-common</artifactId>
-			<version>develop-SNAPSHOT</version>
+			<version>f~groups-SNAPSHOT</version>
 		</dependency>
 		<dependency>
 			<groupId>org.openjfx</groupId>
diff --git a/src/main/java/envoy/client/data/LocalDB.java b/src/main/java/envoy/client/data/LocalDB.java
index 256e701..a9b0665 100644
--- a/src/main/java/envoy/client/data/LocalDB.java
+++ b/src/main/java/envoy/client/data/LocalDB.java
@@ -3,7 +3,9 @@ package envoy.client.data;
 import java.util.*;
 
 import envoy.data.*;
-import envoy.event.*;
+import envoy.event.GroupResizeEvent;
+import envoy.event.MessageStatusChangeEvent;
+import envoy.event.NameChangeEvent;
 
 /**
  * Stores information about the current {@link User} and their {@link Chat}s.
@@ -19,7 +21,7 @@ import envoy.event.*;
 public abstract class LocalDB {
 
 	protected User								user;
-	protected Map<String, User>					users			= new HashMap<>();
+	protected Map<String, Contact>				users			= new HashMap<>();
 	protected List<Chat>						chats			= new ArrayList<>();
 	protected IDGenerator						idGenerator;
 	protected Cache<Message>					messageCache	= new Cache<>();
@@ -69,12 +71,12 @@ public abstract class LocalDB {
 	 *         user names as keys
 	 * @since Envoy Client v0.2-alpha
 	 */
-	public Map<String, User> getUsers() { return users; }
+	public Map<String, Contact> getUsers() { return users; }
 
 	/**
 	 * @param users the users to set
 	 */
-	public void setUsers(Map<String, User> users) { this.users = users; }
+	public void setUsers(Map<String, Contact> users) { this.users = users; }
 
 	/**
 	 * @return all saved {@link Chat} objects that list the client user as the
@@ -161,7 +163,7 @@ public abstract class LocalDB {
 	 * @since Envoy Client v0.1-beta
 	 */
 	public void replaceContactName(NameChangeEvent event) {
-		chats.stream().map(Chat::getRecipient).filter(c -> c.getID() == event.getId()).findAny().ifPresent(c -> c.setName(event.get()));
+		chats.stream().map(Chat::getRecipient).filter(c -> c.getID() == event.getID()).findAny().ifPresent(c -> c.setName(event.get()));
 	}
 
 	/**
@@ -180,10 +182,10 @@ public abstract class LocalDB {
 			.ifPresent(group -> {
 				switch (event.getOperation()) {
 					case ADD:
-						group.getMemberIDs().add(event.get());
+						group.getContacts().add(event.get());
 						break;
 					case REMOVE:
-						group.getMemberIDs().remove(event.get());
+						group.getContacts().remove(event.get());
 						break;
 				}
 			});
diff --git a/src/main/java/envoy/client/net/Client.java b/src/main/java/envoy/client/net/Client.java
index a61d6de..9ae69fb 100644
--- a/src/main/java/envoy/client/net/Client.java
+++ b/src/main/java/envoy/client/net/Client.java
@@ -5,6 +5,7 @@ import java.io.IOException;
 import java.net.Socket;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Set;
 import java.util.logging.Logger;
 
 import javax.naming.TimeLimitExceededException;
@@ -15,7 +16,8 @@ import envoy.client.data.LocalDB;
 import envoy.client.event.SendEvent;
 import envoy.data.*;
 import envoy.event.*;
-import envoy.event.ContactOperationEvent.Operation;
+import envoy.event.contact.ContactOperationEvent;
+import envoy.event.contact.ContactSearchResult;
 import envoy.util.EnvoyLog;
 import envoy.util.SerializationUtils;
 
@@ -40,9 +42,9 @@ public class Client implements Closeable {
 	private boolean		online;
 
 	// Asynchronously initialized during handshake
-	private volatile User		sender;
-	private volatile Contacts	contacts;
-	private volatile boolean	rejected;
+	private volatile User					sender;
+	private volatile Set<? extends Contact>	contacts;
+	private volatile boolean				rejected;
 
 	// Configuration and logging
 	private static final ClientConfig	config	= ClientConfig.getInstance();
@@ -67,7 +69,6 @@ public class Client implements Closeable {
 	public void performHandshake(LoginCredentials credentials, Cache<Message> receivedMessageCache)
 			throws TimeLimitExceededException, IOException, InterruptedException {
 		if (online) throw new IllegalStateException("Handshake has already been performed successfully");
-
 		// Establish TCP connection
 		logger.info(String.format("Attempting connection to server %s:%d...", config.getServer(), config.getPort()));
 		socket = new Socket(config.getServer(), config.getPort());
@@ -77,8 +78,7 @@ public class Client implements Closeable {
 		receiver = new Receiver(socket.getInputStream());
 
 		// Register user creation processor, contact list processor and message cache
-		receiver.registerProcessor(User.class, sender -> this.sender = sender);
-		receiver.registerProcessor(Contacts.class, contacts -> this.contacts = contacts);
+		receiver.registerProcessor(User.class, sender -> { this.sender = sender; contacts = sender.getContacts(); });
 		receiver.registerProcessor(Message.class, receivedMessageCache);
 		receiver.registerProcessor(HandshakeRejectionEvent.class, evt -> { rejected = true; EventBus.getInstance().dispatch(evt); });
 
@@ -92,7 +92,7 @@ public class Client implements Closeable {
 
 		// Wait for a maximum of five seconds to acquire the sender object
 		long start = System.currentTimeMillis();
-		while (sender == null || contacts == null) {
+		while (sender == null) {
 
 			// Quit immediately after handshake rejection
 			// This method can then be called again
@@ -151,8 +151,9 @@ public class Client implements Closeable {
 		// Process contact searches
 		receiver.registerProcessor(ContactSearchResult.class, EventBus.getInstance()::dispatch);
 
-		receiver.registerProcessor(Contacts.class,
-				contacts -> EventBus.getInstance().dispatch(new ContactOperationEvent(contacts.getContacts().get(0), Operation.ADD)));
+		receiver.registerProcessor(Contact.class,
+				contacts -> EventBus.getInstance()
+					.dispatch(new ContactOperationEvent(contacts.getContacts().iterator().next(), ElementOperation.ADD)));
 
 		// Process group size changes
 		receiver.registerProcessor(GroupResizeEvent.class, evt -> { localDB.updateGroup(evt); EventBus.getInstance().dispatch(evt); });
@@ -218,10 +219,10 @@ public class Client implements Closeable {
 	 *         user names as keys
 	 * @since Envoy Client v0.2-alpha
 	 */
-	public Map<String, User> getUsers() {
+	public Map<String, Contact> getUsers() {
 		checkOnline();
-		Map<String, User> users = new HashMap<>();
-		contacts.getContacts().forEach(u -> users.put(u.getName(), u));
+		Map<String, Contact> users = new HashMap<>();
+		contacts.forEach(u -> users.put(u.getName(), u));
 		return users;
 	}
 
@@ -245,10 +246,10 @@ public class Client implements Closeable {
 	/**
 	 * Sets the client user which is used to send messages.
 	 *
-	 * @param sender the client user to set
+	 * @param clientUser the client user to set
 	 * @since Envoy Client v0.2-alpha
 	 */
-	public void setSender(User sender) { this.sender = sender; }
+	public void setSender(User clientUser) { this.sender = clientUser; }
 
 	/**
 	 * @return the {@link Receiver} used by this {@link Client}
@@ -265,11 +266,11 @@ public class Client implements Closeable {
 	 * @return the contacts of this {@link Client}
 	 * @since Envoy Client v0.3-alpha
 	 */
-	public Contacts getContacts() { return contacts; }
+	public Set<? extends Contact> getContacts() { return contacts; }
 
 	/**
 	 * @param contacts the contacts to set
 	 * @since Envoy Client v0.3-alpha
 	 */
-	public void setContacts(Contacts contacts) { this.contacts = contacts; }
+	public void setContacts(Set<? extends Contact> contacts) { this.contacts = contacts; }
 }
diff --git a/src/main/java/envoy/client/net/UserStatusChangeProcessor.java b/src/main/java/envoy/client/net/UserStatusChangeProcessor.java
index 761f7d8..9120662 100644
--- a/src/main/java/envoy/client/net/UserStatusChangeProcessor.java
+++ b/src/main/java/envoy/client/net/UserStatusChangeProcessor.java
@@ -3,6 +3,7 @@ package envoy.client.net;
 import java.util.function.Consumer;
 
 import envoy.client.data.LocalDB;
+import envoy.data.User;
 import envoy.event.EventBus;
 import envoy.event.UserStatusChangeEvent;
 
@@ -26,7 +27,7 @@ public class UserStatusChangeProcessor implements Consumer<UserStatusChangeEvent
 
 	@Override
 	public void accept(UserStatusChangeEvent evt) {
-		localDB.getUsers().values().stream().filter(u -> u.getID() == evt.getID()).findFirst().get().setStatus(evt.get());
+		localDB.getUsers().values().stream().filter(u -> u.getID() == evt.getID()).map(User.class::cast).findFirst().get().setStatus(evt.get());
 		EventBus.getInstance().dispatch(evt);
 	}
 }
diff --git a/src/main/java/envoy/client/ui/ChatSceneController.java b/src/main/java/envoy/client/ui/ChatSceneController.java
index 1b68322..1882fba 100644
--- a/src/main/java/envoy/client/ui/ChatSceneController.java
+++ b/src/main/java/envoy/client/ui/ChatSceneController.java
@@ -3,15 +3,16 @@ package envoy.client.ui;
 import java.io.IOException;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+import java.util.stream.Collectors;
 
 import envoy.client.data.Chat;
 import envoy.client.data.LocalDB;
 import envoy.client.event.MessageCreationEvent;
 import envoy.client.net.Client;
 import envoy.client.net.WriteProxy;
+import envoy.data.Contact;
 import envoy.data.Message;
 import envoy.data.MessageBuilder;
-import envoy.data.User;
 import envoy.event.EventBus;
 import envoy.event.MessageStatusChangeEvent;
 import envoy.event.UserStatusChangeEvent;
@@ -26,7 +27,7 @@ import javafx.scene.input.KeyEvent;
  * Project: <strong>envoy-client</strong><br>
  * File: <strong>ChatSceneController.java</strong><br>
  * Created: <strong>26.03.2020</strong><br>
- * 
+ *
  * @author Kai S. K. Engelbart
  * @since Envoy Client v0.1-beta
  */
@@ -39,7 +40,7 @@ public final class ChatSceneController {
 	private ListView<Message> messageList;
 
 	@FXML
-	private ListView<User> userList;
+	private ListView<Contact> userList;
 
 	@FXML
 	private Button postButton;
@@ -95,13 +96,13 @@ public final class ChatSceneController {
 		this.writeProxy	= writeProxy;
 
 		// TODO: handle offline mode
-		userList.setItems(FXCollections.observableList(client.getContacts().getContacts()));
-		// userList.getItems().addAll(localDB.getChats().stream().map(Chat::getRecipient).collect(Collectors.toList()));
+		// userList.setItems(FXCollections.observableList(client.getContacts()));
+		userList.setItems(FXCollections.observableList(localDB.getUser().getContacts().stream().collect(Collectors.toList())));
 	}
 
 	@FXML
 	private void userListClicked() {
-		final User user = userList.getSelectionModel().getSelectedItem();
+		final Contact user = userList.getSelectionModel().getSelectedItem();
 		if (user != null && (currentChat == null || user.getID() != currentChat.getRecipient().getID())) {
 			contactLabel.setText(user.getName());
 
diff --git a/src/main/java/envoy/client/ui/Startup.java b/src/main/java/envoy/client/ui/Startup.java
index c392281..e007058 100644
--- a/src/main/java/envoy/client/ui/Startup.java
+++ b/src/main/java/envoy/client/ui/Startup.java
@@ -14,6 +14,7 @@ import envoy.client.net.Client;
 import envoy.client.net.WriteProxy;
 import envoy.client.ui.container.LoginDialog;
 import envoy.data.Message;
+import envoy.data.User;
 import envoy.data.User.UserStatus;
 import envoy.exception.EnvoyException;
 import envoy.util.EnvoyLog;
@@ -28,7 +29,7 @@ import javafx.stage.Stage;
  * Project: <strong>envoy-client</strong><br>
  * File: <strong>Startup.java</strong><br>
  * Created: <strong>26.03.2020</strong><br>
- * 
+ *
  * @author Kai S. K. Engelbart
  * @since Envoy Client v0.1-beta
  */
@@ -121,7 +122,12 @@ public final class Startup extends Application {
 			writeProxy.flushCache();
 		} else
 			// Set all contacts to offline mode
-			localDB.getUsers().values().stream().filter(u -> u != localDB.getUser()).forEach(u -> u.setStatus(UserStatus.OFFLINE));
+			localDB.getUsers()
+				.values()
+				.stream()
+				.filter(u -> u instanceof User && u != localDB.getUser())
+				.map(User.class::cast)
+				.forEach(u -> u.setStatus(UserStatus.OFFLINE));
 	}
 
 	/**
@@ -164,5 +170,6 @@ public final class Startup extends Application {
 		}
 	}
 
+	@SuppressWarnings("javadoc")
 	public static void main(String[] args) { launch(args); }
 }
diff --git a/src/main/java/envoy/client/ui/UserListCell.java b/src/main/java/envoy/client/ui/UserListCell.java
index f62f98a..cfc1fdf 100644
--- a/src/main/java/envoy/client/ui/UserListCell.java
+++ b/src/main/java/envoy/client/ui/UserListCell.java
@@ -1,5 +1,6 @@
 package envoy.client.ui;
 
+import envoy.data.Contact;
 import envoy.data.User;
 import javafx.scene.control.Label;
 import javafx.scene.control.ListCell;
@@ -9,22 +10,24 @@ import javafx.scene.layout.VBox;
  * Project: <strong>envoy-client</strong><br>
  * File: <strong>UserListCell.java</strong><br>
  * Created: <strong>28.03.2020</strong><br>
- * 
+ *
  * @author Kai S. K. Engelbart
  * @since Envoy Client v0.1-beta
  */
-public class UserListCell extends ListCell<User> {
+public class UserListCell extends ListCell<Contact> {
 
 	/**
 	 * {@inheritDoc}
 	 */
 	@Override
-	protected void updateItem(User user, boolean empty) {
-		super.updateItem(user, empty);
-		if (!empty && user != null) {
-			final Label	name	= new Label(user.getName());
-			final Label	status	= new Label(user.getStatus().toString());
-			setGraphic(new VBox(name, status));
+	protected void updateItem(Contact contact, boolean empty) {
+		super.updateItem(contact, empty);
+		if (!empty && contact != null) {
+			final Label name = new Label(contact.getName());
+			if (contact instanceof User) {
+				final Label status = new Label(((User) contact).getStatus().toString());
+				setGraphic(new VBox(name, status));
+			} else setGraphic(new VBox(name));
 		}
 	}
 }
diff --git a/src/main/java/envoy/client/ui/container/LoginDialog.java b/src/main/java/envoy/client/ui/container/LoginDialog.java
index 0d9f034..b523849 100644
--- a/src/main/java/envoy/client/ui/container/LoginDialog.java
+++ b/src/main/java/envoy/client/ui/container/LoginDialog.java
@@ -65,7 +65,7 @@ public class LoginDialog extends JDialog {
 
 	private static final ClientConfig	config				= ClientConfig.getInstance();
 	private static final Logger			logger				= EnvoyLog.getLogger(LoginDialog.class);
-	private static final long			serialVersionUID = 0L;
+	private static final long			serialVersionUID	= 0L;
 
 	/**
 	 * Displays a dialog enabling the user to enter their user name and password.
@@ -143,7 +143,7 @@ public class LoginDialog extends JDialog {
 			try {
 				// Try entering offline mode
 				localDB.loadUsers();
-				User clientUser = localDB.getUsers().get(credentials.getIdentifier());
+				User clientUser = (User) localDB.getUsers().get(credentials.getIdentifier());
 				if (clientUser == null) throw new EnvoyException("Could not enter offline mode: user name unknown");
 				client.setSender(clientUser);
 				JOptionPane.showMessageDialog(null,