Maybe this is what the Actvity tab is supposed to accomplish?
Correct.
Where is this documented? Is this a new plugin I don't see in my CB updater?
Simply install CB GroupJive, CB Gallery, CB Activity, and CB Auto Actions. Once done publish the following in CB Auto Actions > System Actions.
CB Groupjive - Gallery
CB Groupjive - Stream
That's it. You should now have an Activity and Gallery tab available in your groups. In a later release these will just be parameters directly in the group and automatically integrated with CB GroupJive.