Replying to Pulled over and someone has weed?

Quick question, I am going on a road trip in a couple of weeks with a group of people.  We will be driving from MI to FL.  There is a chance that someone who is going with us might try and sneak some weed along.  We have told them not to bring it, but I wouldn't put it past them to try and pull it by us.  If we were to get pulled over and the cops found it, what would happen?  Would we all get in trouble or just the person that has it?  Could we all face possession charges?

It's obviously a situation I am going to do everything I can to avoid, I just want to know what the worst case scenario would be were something to happen.  Thanks.