This would mean it was both intransitive and transitive. So whilst people may be switching rapidly between the two concepts, you can't combine them without a problem. My view is that Group is a word which is used with different meaning in different contexts. Treat groups as sets. Matthew chose option 2.
Lets be clear.
I choose 2 for group in this particular context. Give me another context, I might make a different choice. There are some important principles to understand here about fusion. Let me explain. So for example, a glass of water is a fusion of water molecules.