Secret club posts quit appearing on the forum index, but there's a way to find them from there.
Club members can be club admins which lets them invite/kick users.
Clubs can have up to 3 posts at a time. Any member can make a new post, which locks an older post. Only the owner can make more than one post at a time.
It's easier to request access to a secret club.
These work in reverse -- instead of not belonging by default and needing an invitation, you belong by default and can be blocked from the club.
The other rules still apply, except the way that new posts get made is a bit different.
I'm not totally sure on the 3 per club rule. I'd like clubs to have no limit but I feel like that would rapidly serve to split the forums and kill off public activity. Maybe staying at 1 is better. Maybe 3 is a good compromise, but I'll have to work on the mechanics of making new posts / locking old ones a bit more.