I would think that the presence of a plus sign would indicate a member group. No plus, single set. Creating a group could be automatically minus sign tree'd, then a collaps when room is needed. A quick glance at the tree would indicate contents of a group.

But I guess it then becomes the discoverabilty of nested objects. Maybe selecting an object would indicate where it lies in the tree. Kind of like now, when you select an object, the selection indicator in the object list highlights and reminds me the nameset it belongs to......?