My Groups activity in the river

I'd like to get a new "Subscription" tab in the river activity.

The idea is to merge the friends tab in the river with all the group activities the user is member of, so every activity the user wants to follow (people/friends and groups are avalieble in one only river tab).

I've seen the mt_activity_tabs plugin, but it needs input from the user...

I can create a new tab with no problem, but, how to get all the activity for user's groups?

Thanks.