Make use of the 'contacts' table so users can add other users to their contact list. From this contact list there can be direct access to send a message for example.
for examples:
/user/act/addcontact.php?id={$user_id)
/user/act/removecontact.php?id=($user_id)
/mail/contactlist.php
> foreach contact > /mail/sendmessage.php