Sorry, it has to do with the TABLE_PREFIX you're using. The query needs some adjustment.. try:
PHP Code:
$recipients=$DB_site->query("SELECT reputation.userid,user.username FROM " . TABLE_PREFIX . "reputation AS reputation
LEFT JOIN " . TABLE_PREFIX ."user AS user ON user.userid=reputation.userid
WHERE reputation.whoadded='".$bbuserinfo['userid']."' ORDER BY dateline DESC LIMIT 0,10");