The this is the query you want.
PHP Code:
$users = $DB_site->query("
SELECT
$locationfieldselect forum.forumid, forum.title AS forumtitle,
user.username, user.userid, user.invisible, user.showemail, user.receivepm, user.lastactivity, user.lastvisit
FROM moderator
LEFT JOIN user ON (user.userid = moderator.userid)
LEFT JOIN forum ON (forum.forumid = moderator.forumid)
LEFT JOIN userfield ON (userfield.userid = user.userid)
WHERE forum.active = 1
ORDER BY user.username ASC, forum.displayorder ASC
");