I think you want something like this:
Code:
$groupid = 9;
$veteran_members = $db->query("
SELECT userid, usergroupid,
IF (displaygroupid=0, usergroupid, displaygroupid) AS displaygroupid,
username, lastpostid, posts, joindate
FROM " . TABLE_PREFIX . "user AS user
WHERE (FIND_IN_SET($groupid, membergroupids) OR user.usergroupid = $groupid)
ORDER BY joindate ASC
LIMIT $count
");