People can join. People can invite. With this plugin admin can manually add members so there's no reason to do that. 

My plugin is not for you then and I really don't believe IPS will ever do that.

I doubt moving the content from a forum to a club would remove it from their view, unless you made the group private.  If the group is public then nothing should change except the URL.

Yes, it depends on the club type.

• For “Public”, the content would be visible for everyone
• for “Open” participants of discussion would see their posts but not be able to answer without finding the join button first. (see my feature request)
• for “closed” and “private” clubs the content would disappear for almost anyone but the members of those clubs

So I do understand the request to add members from moved content to a club. But you are also forcing members into a club without their consent, which they might not like.