BP_Messages_Thread::check_access( int $thread_id, int $user_id )

	public static function check_access( $thread_id, $user_id = 0 ) {

		if ( empty( $user_id ) ) {
			$user_id = bp_loggedin_user_id();
		}

		$recipients = self::get_recipients_for_thread( $thread_id );

		if ( isset( $recipients[ $user_id ] ) && 0 == $recipients[ $user_id ]->is_deleted ) {
			return $recipients[ $user_id ]->id;
		} else {
			return null;
		}
	}