Proper Comma Construction?

[kaotic]

Here's the code that I'm working with:

$GetUser = $DB_site->query("select s.*,u.* from ".TABLE_PREFIX."session s
left join ".TABLE_PREFIX."user u on (u.userid = s.userid) where s.location like '%test.php%'");
	while($UserIN = $DB_site->fetch_array($GetUser)){
	if(!$store_c[$UserIN['userid']]){
		if($UserIN['userid']){
		$activeusers .= "<a href='member.php?u={$UserIN['userid']}'>{$UserIN['username']}</a>, ";
		}
	}
}

I'm using it to contruct a users online list, but I'm having a problem with comma placements. Using the code above, a comma is placed once after each users name, including the last one to be listed... so it ends up looking like:

user1, user2, user3,

Can anyone help me rid myself of the final comma? Assistance would be most appreciated.