If you don't want to mess with the module code, you could put the following sql statement into the reports module (dnn4)
something like:
select u.Firstname, u.LastName from usersonline as UO inner join users as U on UO.UserID=U.Userid where UO.PortalID=0
(change the portalid to your portalID in the example)
Or I have a module that is similar to the text.html module but allows data access and you could put that query into it and then format the output anyway you like.
mj