How to put own roommates in University Housing?

If you have GT you can use the club system to choose your roomies. You can use the same method for sororities and fraternities.

Make a club and add the other sims you want as roomies. Set the Affiliation of the dorm to the club name. You can choose a single gender if you want a Greek House too.

Only sims in the club should be able to move in to that lot.

You could set up the club as teens.