This creates a new group node by adding a copy of the PrototypeGroup data to the model, which must support the 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.

Reference

CommandHandler Class
CommandHandler Members