A group is just a “special” class of component instance (whose definition is hidden from the Component Browser panel, and when there are more than 1 instance manually opening the instance for edit creates a new unique clone definition for the instance being edited.)
So groups, component instances and images all are instances of a “mother” definition.
The solution with the C API is like that given above. When you need functionality from related classes of objects you will need to cast your object to that other class. (So cast your SUGroupRef
to an SUComponentInstanceRef
and then call the desired function.)