If one of my roommates sells weed, what legal trouble can the rest of us have if he gets busted?

I don't know what "randomly assigned" means here - are you in a college dorm?  Of course, you can get in trouble if your roommate sells weed or any other illegal substance.  There is a difference between being questioned, arrested, and convicted.  You could easily be questioned by law enforcement about your roommate's activity.  You could be arrested if illegal drugs are found in your room, apartment, home, or car.  You would then have to hire a lawyer and defend yourself.  You might win; you might not, depending on the circumstances.

You don't need any of this trouble.  Get rid of the roommate.

And, let me give you the advice I give everyone about talking to the police, and I mean everyone.  Unless you are the person making a report, do not talk to the police.  Ever.  Most people think explaining things to the police will get you off.  Almost all of time, it won't get you off.  It will make everything worse.  I know it's scary.  I know it costs money to get a lawyer.  Exercise your constitutional rights anyway - tell the police you want a lawyer and then don't talk until a lawyer arrives.  If they arrest you, they were going to arrest you anyway, I promise.

And, by the way, don't start thinking you can hide things from the police.  You can't even hide this question and answer.  If the police are sufficiently motivated, they will find everything.  Hiding things just makes your life miserable moment by moment.  Do the right thing from the beginning - you and your friends are not involved in selling illegal substances.  Tell the roommate to move.  If he or she will not voluntarily move, go to the landlord or the school authorities and get him or her moved.  You owe nothing to someone who would put you in this danger.