Firstly I think the Groups feature is excellent, and shaping up fast, thanks to the efforts of @tgxworld and others.
One suggestion - I think the “Topics” and “Activity” tabs need re-thinking on the group page.
These tabs show all topics and all activity by group members. This doesn’t make sense in the use case where groups are used to control access to special categories.
The content of these special categories constitutes the “activity” of the group (and in my case I’d want this activity to be hidden from forum users unless they are a member of the group).