This has come up before here and here. Other platforms address it with category visibilty settings for
- Public - can see category and topics
- Private - can see category only (including description but not topics)
- Hidden - only group members see category and topics
Adding a Private visibilty setting would help people know a category exists, how to join via category description, etc. Would be nice not to have to use group membership to make categories suddenly appear after joining.
Showing titles of posts but not content would be trickier.