Nice work Iñaki! I tested the diff file today and it seems to work well. Since I've been doing some work with usability issues, my eye happened to catch
that you are using the pushbuttons to add and remove folks from the lists. Thus the interface for:
is slightly different than the interface for:
http://localhost/moodle/19stable/group/members.php?group=1 (I'll attach a screenshot of the groups interface)
I think they should be consistent. I also wonder if the available members in groups should be on the left. The way you set it up of having available members first (on the left - at least for cultures that read from left to right) seems more intuitive as you start with everyone and then show who is selected. I will raise this as a usability question for Laia in the GSOC and add her and David to the watchers of this issue.
Peace - Anthony