The SOP Editor allows you to select, create, and edit geometry in SOPs.
To open the SOP Editor, right click on any SOP and select Model Geometry. This will lock the node and open up the SOP Editor in a floating pane. For the Group SOP, the menu item is Select Geometry. The SOP will not be locked, but you will be able to select geometry and transfer the selection to the Group SOP.
See also Group SOP