RB.ReviewGroup can now add users to the group or remove users from the

group through the new addUser/removeUser functions.
This works much in the same way as watched object tracking. We have a

proxy object, GroupMember, that we can call save() or destroy() on, and

it will invoke the POST or DELETE request with the data needed for

manipulating that membership.

Unit tests pass.
Tested manually with my new My Account page change.