Normal compliment for a utility helicopter in the US Army is three. Two pilots, one designated as PIC (Pilot in Command), and one just designated as pilot, and one crew chief. The "crew chief" name is a misnomer in the Army since they guy typically works alone and is generally of lower rank, PFC, SP4, or SGT. The crew chief is responsible for aircraft maintenance and also has crew duties in flight.
In combat, there will often be a door gunner assigned to the aircraft as well. The door gunner could be a junior mechanic, or an infantry soldier assigned from a garrison unit. The crew chief will man the second machine gun when necessary.