Well its not exactly possible as their is a column called inforum on the user table so when a user enters a forum this column is updated in the table.
Guests don't have entires in the user table so they wont show up.
open forumdisplay.php
look for
PHP Code:
$datecut = $ourtimenow - $cookietimeout;
below it put
PHP Code:
$guestsnum = $DB_site->query_first("SELECT COUNT(*) AS total FROM session WHERE LOCATE('forumid=$foruminfo[forumid]', location)>0 AND LOCATE('forumdisplay.php', location)>0 AND userid=0 AND lastactivity > $datecut");
use $guestnum[total] in the forumdisplay template