The Scrollbar class has no objects and attributes itself. It just connects a proportional gadget and two button gadgets with approriate imagery to make up a scrollbar.
Since Scrollbar class is a subclass of Group class, every attribute and method is passed through to all of its children. Thus, you can talk and listen to a scrollbar as if it was just a single prop gadget.
You can use the attribute MUIA_Group_Horiz as with any other group to determine if the scrollbar should be horizontal or vertical. By default, a vertical scrollbar is generated.