There shouldn't be a space before unless I am mistaken, but if you're looking for the general area the commas are placed when eval'ing the templates. For the whos online on the index it would be located in the index.php file and here's the general line that you would be looking for.
PHP Code:
eval("\$activeusers .= \", ".gettemplate('forumhome_loggedinuser')."\";");
If you look at the $activeusers variable to the right you see the comman then space before the gettemplate function is preformed that is where you comma and space are added.
Regards,
g-force2k2