Sounds interesting.
Spec wise I think you need to nail down what exactly you need out of these groups.
- group creation
can anyone create a group? is there moderation around new groups - group visibility
hidden/private groups versus public groups - UI to display these groups and their messages
Do you want group messages to be visible in the #latest list?
Do you want to segrate regular posts with group posts?
Where will you display the list of groups?