[Admin]

This is how I did it, only adds one query to the index page and the query is quite fast as well.

I added this somewhere:

// Users browsing each forum
$getforumbrowsers = $DB_site->query('
	SELECT COUNT(*) AS count, inforum
	FROM user
	WHERE lastactivity>'.($ourtimenow - $cookietimeout).'
	AND lastvisit<>lastactivity
	GROUP BY inforum
');
$forumbrowsers = array();
while ($getforumbrowser = $DB_site->fetch_array($getforumbrowsers)) {
	$forumbrowsers[$getforumbrowser['inforum']] = $getforumbrowser['count'];
}

Then stuck this inside the makeforumbit() function:

		global $forumbrowsers;
		$browsers = $forumbrowsers[$forum['forumid']];
		if (intval($browsers) < 1) {
			$browsers = 0;
		}

Then used $browsers in the template.