go to the forum front page,
scroll to the bottom, youll see that we already have a "what's going on box"
this box has three sections
Currently active users: [count] which is then divided into ([count] members and [count] guests)
it tells you what the most users online ever was, and the names of the currently active users
it tells you forums statistics, like amount of threads and amount of posts,
and who the newest registered member is
also any birthdays for registered members
that's on the main page.
go into a topic and the box changed to show you the number of members active in a single topic, members guests and names of active members.
nice idea, and kudos for David for implementing it about five years before it was asked for!
Oh, and active members counts as those currently online,
or those that have been online in the recent past, (probably ten minutes, though David could say for sure)