This creates a new group node by adding a copy of the PrototypeGroup data to the model, which must support the Northwoods.GoXam.Model.ISubGraphModel interface. Each of the selected nodes for which Part.CanGroup is true is made a member of that new group node. If all of the selected groupable nodes were members of a pre-existing group node, the new group node also becomes a member of that pre-existing group. The new group node becomes the only selected part. This also raises the SelectionGrouped. All of the changes are performed in an undoable edit.