Penso che tu possa:
SELECT
u.id AS user_id,
CURRENT_TIMESTAMP AS granted_at,
(SELECT id FROM posts WHERE topic_id = YOUR_TOPIC_ID AND post_number = 1) AS post_id
FROM users u
INNER JOIN group_users gu ON gu.user_id = u.id
INNER JOIN groups g ON g.id = gu.group_id
WHERE g.name = 'beta-testers'
Sostituisci YOUR_TOPIC_ID
E penso che debba essere spuntata l’opzione «Query targets posts».
Fammi sapere se funziona.