Sure, you won't get the entire car but you'll get a row. Each car is usually comprised of several different parties actually.
Most seating at SciFi is in the cars; we've dined there many times and never even considered specifically requesting them as most are seated there anyways and we always have been but you're welcome to tell them at check in that you want to be sure and be seated in a car.
I don't know about peak season but my mother and I went back in Feb. (value season) together and were seated behind a family of 4 in a car. If it's really important to you, I would request it at the podium.
I recently made ADRs for the Sci-Fi and as part of the reservation process she booked us into a car. In our case, since it was a last minute reservation, they had to break us into two seperate cars but my older kids don't care.
Lou7401, I did this too and they said not to worry, the party will be seated together. And we were. So you should not be in separate cars; they just have to book it that way.
Just to add, in case sitting in a car with strangers weirds anyone out, you are not actually sitting WITH them. It's not weird at all. Everyone is facing forward so you only see the backs of their heads.