Redesign group membership interface

The current interface for adding/removing members of a group is really clumsy; I find it challenging to correctly select the users I want in my group in the large selection box.

I would much prefer two selection boxes, one with available users and one with group members. In between the boxes there would be the option to add a user from the available box to the group or remove a user from the group back to the available box.... I'm open to any new interface ideas really... just my 2 cents.