ho risolto da solo ( meglio così ), se a qualcuno può essere d'aiuto posto la query:
Codice PHP:
SELECT u.*, g.*, o.*, COUNT(pm.read) as num_mp
FROM users AS u
INNER JOIN groups AS g ON g.group_id = u.groups
LEFT JOIN online AS o ON o.user_id = u.user_id
LEFT JOIN mp AS pm ON pm.receiver_id = u.user_id
WHERE pm.read = 0 AND
u.user_id = "{id_utente_cookie}" AND
u.user_password = '"{pass_utente_cookie}"'")