When a user is online, the feild "IsOnline" in the aspnet_User table is set to 1/true. By default, the only way you can get a list of online users is retrieving ALL the users and see if they are online. What you could do is build a custom stored procedure that returns all users with IsOnline set to 1/true. SubSonic can help you generate a wrapper for any sproc you create.
Contact me for more info.
aim : SN 0291FZ4Q v2
yahoo: theonlylawislove
msn: theonlylawislove@yahoo.com