Well, if you want to join dorms with someone, then you have to go through the process of agreeing with them, setting up the dorm together, then showing me this finished dorm through, of course, dorm creation.
So, new students can only be in a shared dorm if they agree with someone else (be it a new or old student) to share that dorm. If a student that already has a dorm decides to share a dorm, then he or she will move out of their old dorm once the shared dorm has been constructed.
BASICALLY, you need a roommate. That's the number one step in getting a shared dorm, and you can't have an even number for your room unless you have one. Hope I made things clear.