I think something like this could do it:
WITH book_club_first_posts AS (
SELECT
p.topic_id,
p.user_id,
MIN(p.id) AS post_id
FROM posts p
JOIN topics t ON t.id = p.topic_id
WHERE t.category_id = 5 -- replace with the category_id of your book club
AND p.deleted_at ISNULL
AND t.deleted_at ISNULL
AND p.post_type = 1
AND p.post_number <> 1
AND p.user_id > 0
GROUP BY p.topic_id, p.user_id
)
SELECT bcfp.user_id, bcfp.post_id, p.created_at granted_at
FROM book_club_first_posts bcfp
JOIN posts p ON p.id = bcfp.post_id
WHERE (:backfill OR p.id IN (:post_ids))