Allow users to set per-group notification preferences. My work recently created several umbrella groups that contain 100's of people and these groups get added as collaborators to announcement type documents. These documents generate a lot of noise with comments, etc. Now I have to choose whether to get flooded with notifications or turn them off. I would prefer to disable notifications of just the documents associated with these big umbrella groups, not the notifications for documents to which my account has been specifically added as a collaborator.