title: django unable to add user to group
body: <p>I have this user class:</p>
  
  <pre><code>class Parent(User):
      contact_means = models.IntegerField()
      is_active = False
      is_staff = False
      is_superuser = False
      #parent_id = models.AutoField()
      class Meta:
          db_table = 'parent'
  </code></pre>
  
  <p>During adding of Parent in my admin page, I want to auto add this parent into a user group:</p>
  
  <pre><code>class ParentAdmin(admin.ModelAdmin):
      def save_model(self,request,modelObj,modelFormIns, change):
          if not change: #new entry
              #save regular parent and user
              parentGroup = Group.objects.get(pk=1)
  
              modelObj.save()
              modelObj.groups.add(parentGroup)
              #parentGroup.save()
              #save a record under django-registration
              profile_callback = None
              RegistrationProfile.objects.create_inactive_user_from_fk(user = modelObj,send_email = True)
  
          else:
              modelObj.save()
  </code></pre>
  
  <p>This doesn't work! no user is added to the group. why?</p>