groups: port missing functionality

* Implement user group manage function

* Add filter on list of group that user can join.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
shouldn't this be `'Add user'`?

![image](https://cloud.githubusercontent.com/assets/189798/4925056/fdaabfd4-6527-11e4-9723-111df48ab20b.png)