[Brother Malachi]

I guess you can run a query, grab all the users whose referid isn't empty, then sort it through an array.

PHP Code:

$users_rep_query = $db->query_read("SELECT referrerid FROM  " . TABLE_PREFIX . "user WHERE  referrerid IS  NOT  NULL");

$users_rep_sort = array();
while ( $users_rep_array = $vbulletin->db->fetch_array($users_rep_query) ) {
    $users_rep_sort[$users_rep_array['referrerid']]++;
}
arsort($users_rep_sort); 


The above will give you an array with userID's, sorted by # of referrals.
You just need to take the userID and convert it to a username.