If you don't know what I mean, look at this for a thread: http://community.invisionpower.com/topic/369306-licensing-and-multiple-sites/
and this for a forum: http://community.invisionpower.com/forum/305-pre-sales-questions/
Online users must be split in members, guests, and anonymous/invisible users! (very important!)
Of course the who's online list in the sidebar must also show the amount of anonymous/invisible users. Just try to copy what IPB does.