[Answered ]-Django UserAdmin customization

Here it is:

def get_groups(self, object):
    return u', '.join(g.name for g in object.groups.all()) or 'None'
get_groups.short_description = 'Chosen Groups'

To reduce the number of sql queries you can also override the get_queryset() method:

def get_queryset(self, request):
    qs = super(UserAdmin, self).get_queryset(request)
    return qs.prefetch_related('groups')