Please read GoJS Selection -- Northwoods Software. You could try for a custom Adornment, but it would be easier to just change the Shape.fill or Shape.stroke color when the group is selected, as described in the section GoJS Selection -- Northwoods Software.
The hard part will be in computing non-rectangular intersections, unlike what I did for you in Inside Parts shouldn't move on Group resize - #27 by Abhishek