Strictly speaking, the group is not an object, so it doesn’t have any properties. It’s just a string shared by all the buttons.
I think they did it this way so that the buttons could be distributed into other containing objects, and still remain connected to each other.
I have not seen a way to identify the selected RadioButton object, except by looking at each one in turn. This is simplest if you create a list of the buttons, and loop over that list.
You could loop over every object on the form, but then you’d have to explicitly skip over the objects that were not of interest.