BuddyPress.org

You should be able to do most of this. Depending on how sophisticated you want to be with the auto group join, it may take some coding to make it happen, but I’ve seen others do it.

The second part is a little more tricky depending on how you define not integrating with other users. You can turn off friending and messaging. Basically you can make it so they can only communicate in groups. If you wanted only groups plus, communication directly with people that are members of those groups, that would be harder to implement.