** WIP ** - Join/Leave Groups

** This is currently a work in progress **

I haven't made as much progress lately as I was hoping I would due to some issues with mongoid relations, but I finally got them resolved.  More functionality to come, but I figured I would go ahead and check this in so it is visible:

-Added n-n relationship for user-group
-Store leader_id in groups
-Replace use of "participantNumber" with getting count of members 

Now that we have a many-many relationship setup, extending this to add/remove users to/from groups shouldn't be very difficult.  I'll add to this PR and update the description as I go.