//  Given you have a Pojo called User
private static Map<User> usersLoggedOn = new <User>HashMap();
... ...
 
public static void addToConversation(String login, String password) {
    User user = null;
    if (usersLoggedOn.get(login) == null) {
        user = SomeDataSource.login(login, password);
        if (user != null)
            usersLoggedOn.put(login, user);
    }
}
 
public static Map<User> getUsersLoggedOn() {
    return usersLoggedOn;
}
 
//  SomeDataSource is some persistent storage (could be a database or a file to which you write registered users).