<--(This could be the wrong method but it's the only one I've found/use)-->
Control-click on your group of choice then select "Get info" from the menu that appears. On the "Settings" tab (re)name the "Alias" field, and you're done.
Sadly, this is just giving the group an alias, which im not sure any other client will respect. At the current point in time you cannot rename a group, just alias it.
currently that's the only method, the old "rename group" ability actually created a new group moved everyone, and deleted the old one... it resulted in many empty contact lists