Idiomatically, at least in American English, people are ON teams when you use it this way. You can divide people into teams, but once you have a team, you would say "Who's on your team?"
Barbara, thank you very much for your explanation.
I have found in Oxford Collocations: teamnoun 1 group of people who play a sport together VERB + TEAM -> be in/on team PREP. in/on a/the team -> I'll have you on the first team. team for -> She's in the team for the World Championship.
Like I said, American English. Here, if someone said "20 people are in that team" I'd think it sounded funny. Perhaps a Brit will add what he or she usually hears.
Barbara, Thank you very much for your reply. It's good to know that there are two possibilities. As far as I'm concerned, I'd prefer "in team". IMHO "in" is more logical because a team is a collection/set of elements (players, workers) and you can be in this set/collection, but it's only my point of view. English is very complicate and different from Polish at the sa