Full Idea
Groups may coincide in membership without being identical - extensionality goes.
Gist of Idea
The same members may form two groups
Source
Peter Simons (Parts [1987], 4.9)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.168
A Reaction
Thus an eleven-person orchestra may also constitute a football team. What if a pile of stones is an impediment to you, and useful to me? Is it then two groups? Suppose they hum while playing football? (Don't you just love philosophy?)