Hypnosis is a thing that people do together. It's not like a drug, and it's not like an area effect spell either. It's a type of trancework that alters consciousness in constructive ways. It's good for modifying the reality inside your head.

So, whoever is choosing to do that trancework will be affected by it, and anyone who isn't will not.

This may vary with plural people, depending how closely connected their headmates are. But what you describe is what I'd expect.