На основе примера в таблице event_member
fql:
SELECT uid FROM event_member WHERE eid = ... AND rsvp_status = 'attending'
Полагаю, вы могли бы использовать что-то вроде:
SELECT uid FROM event_member WHERE eid = ... AND rsvp_status = 'attending' AND uid IN (SELECT uid2 FROM friend WHERE uid1 = me())