There is no way of doing this, unless you add some code to the updateuserforum() function, to count all guests in that forum each time the function is called, and store that number somewhere.
OR
You can add code to forumdisplay.php, to get count of guests in the session table, but you will only be able to get users that are currently in the forum listing.
The way it works now, people are listed even if they are just reading a thread in the forum.
Do I make sense?