fix issue adding users to groups

app/models/group.rb

@@ -76,8 +76,8 @@ class Group < ActiveRecord::Base
     GroupUser.where(group_id: trust_group_ids, user_id: user_id).delete_all
 
     if group = Group[name]
-      group_users.build(user_id: user_id)
-      group_users.save!
+      group.group_users.build(user_id: user_id)
+      group.save!
     else
       refresh_automatic_group!(name)
     end

spec/models/group_spec.rb

@@ -47,6 +47,11 @@ describe Group do
 
     Group[:trust_level_1].user_ids.should == []
     Group[:trust_level_2].user_ids.should == [user.id]
+
+    user2 = Fabricate(:coding_horror)
+    user2.change_trust_level!(:regular)
+
+    Group[:trust_level_2].user_ids.sort.should == [user.id, user2.id].sort
   end
 
 end