You are able to request the one roomate through roomate notification (you are able to get more, but you'll get them at check-in).
Getting to check-in early helps your chances.
I think it depends on when you are doing the program. For example- people doing FA were able to start using the system around March, and Fall people couldn't start until aprox June (I would guess Spring/SA people would probably get to start using it around the same time).