Pretty sure they can come onboard one at a time. The first one enters, the second (or more) remain off-board but remain in Platoon Movement status; then in the next MP impulse the next one either enters the board/playing area or remains off-board, though still employing Platoon Movement. That is to say, I do not believe they have to enter on board all in the same impulse (or even the same turn), but must still be part of that Platoon unless released by other factors. As to entering on different turns, I believe yes they can, but must still utilize Platoon Movement status to do so and must conform to turn of entry requirements. Perhaps a Q&A in the offing for clarification?