How Can i get the number of online users in sharepoint 2007?
Extensive explanation, tutorial, etc.: Find Number of Users Currently Logged on to a SharePoint Site
The gist of it is that you don't have OOTB features to show you the number of currently logged in users. You can check IIS for current users, but you don't know whether these are logged on or not. The solution explained in the blog article is to create a delegate control which resides on all pages which counts the number of current sessions. You still don't really have a "live" view as the web is pretty much stateless and it is hard to know whether a user e.g. closed the browser or switched to another tab.