A messaging application user interface has an input element for receiving electronic messages and an output element for displaying electronic messages. The messaging application user interface can be implemented so as to maintain a subset of the plurality of potential message recipients, the subset being determined using a predetermined criterion. The messaging application user interface also can be implemented so as to auto-complete a partially entered address based on the subset of potential message recipients. A set of user-selectable signals that modify auto-completion behavior also can be used with the messaging application user interface. The messaging application user interface can be used in an instant messaging system to allow a user to send instant messages to and exchange comments with a plurality of other users without having to switch among several instant message windows.