I know that it's usually "on team" in AmE. So is it the same with "group". Like if I have to ask someone about his/her group:
I: Which group are you in/on?
Person: I'm not in/on any group.
So should "in" be used or "on" ?
"To be in a group" is the correct form, whether or not it's AmE. We usually don't say "to be on a group".
However, you can use "to be in/on a team". You should look up verbs that collocate with your nouns of choice.