Group membership allows access to a certain forum

[Mangia]

How can I set this up?

I want to create a group, where if members join that group they will have access to a certain forum.

[Videx]

It's easy enough to create a forum and restrict membership to certain usergroups, but I don't think that's what you're asking for.

Social Group forums can be made private - that may come as close as you're going to get.