mysql> SELECT SUM(attendants) AS total_attendants FROM (SELECT meeting_id AS meeting, COUNT(member_id) AS attendants FROM meeting_member GROUP BY (meeting_id)) AS details;
+------------------+
| total_attendants |
+------------------+
|                3 |
+------------------+
1 ROW IN SET (0.00 sec)

mysql> SELECT COUNT(id) AS total_meetings FROM meetings;
+----------------+
| total_meetings |
+----------------+
|              3 |
+----------------+
1 ROW IN SET (0.00 sec)


=====

+------------------+----------------+
| total_attendants | total_meetings |
+------------------+----------------+
|                3 |              3 |
+------------------+----------------+