grammar - Is 'The group criticised John' correct, where John is in the group?

The group = John, Adam and Mary

So this is like 'they criticised John' where they=the group, but that doesn't sound right if John is in they, right?

Answer

The group criticized John.

It is correct whether or not John is (usually) part of the group.

However, it's likely that even when someone knows that John is part of the group, they will assume you mean "everyone in the group except John".

The citizens of the United States are questioning Donald Trump's decisions.

Although Trump is a US citizen, no one is going to assume that Trump is questioning himself.

But that's okay. You don't need to specify it. If that is the case, what you are saying is correct and will be understood.
However, if you want to stress that John also criticized himself, I would stress that fact:

The group, including John, criticized John.

This way, you prevent people from assuming John did not partake in the criticizing.