Best I can suggest is CB Activity integrating with CB GroupJive. It already does this using CB Auto Actions to render an activity stream per group. For notes function it might be better to use a comment stream though. To do this in CB Auto Actions > System Actions make a copy of "CB Groupjive - Stream". Now edit your copy of "CB Groupjive - Stream" in CB Auto Actions > Auto Actions making the following changes.
Action
Stream: Comments
Asset: groupjive.group.[var2_id].notes
Output
Layout:
Code:
return array( 'id' => 'notes', 'title' => \CBLib\Language\CBTxt::T( 'Notes' ), 'content' => $content );
That should be all you'll need to do for a per-group comment stream.