아래처럼 하면 될 것 같아요:
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'
YOUR_TOPIC_ID 부분을 교체해 주세요.
그리고 Query targets posts 옵션이 체크되어 있어야 할 것 같아요.
동작하는지 확인해 보고 알려주세요.