I saw in many US movies, people say
I'm with the fbi, so is it idiomatic to say:
- I'm with the development team.
- I heard you have changed team, are you with the infrastructure team now?
Yes, both of those are perfectly normal expressions that will be used and understood by native English speakers.
You can also use that construction when talking about people, particularly when talking about the current moment
I'm with friends tonight
I'm here with my brother
or in a romantic relationship
I've been with Rebecca for six months now