A real time notification system that tracks, for each user, the logon status of selected co-users of an on-line or network system and displays that information in real time to the tracking user in a unique graphical interface. The invention provides user definable on-line co-user lists, or "buddy lists", that track specific co-users in real-time automatically. A user can create many separate buddy lists of co-users, either with intersecting or disjoint lists of users, and label these buddy lists according to the user's preference. The user can update a buddy list or create new buddy lists whenever necessary. When a user logs on to a system, the user's set of buddy lists is presented to the buddy list system. The buddy list system attempts to match co-users currently logged into the system with the entries on the user's buddy list. Any matches are displayed to the user. As co-users logon and logoff, a user's buddy list is updated to reflect these changes. An indication can also be added to show that a co-user just logged on or just left the system.