So, I have a 5 floor house for 8 Sims and 2 pets, and I felt like adding roommates, but I didn't know how, so I looked it up, and it said about having enough beds for all the Sims in the house plus ex...
Have you changed the Lot Traits in build mode to reflect the number of beds? I think that may be more important that actually having beds when it comes to getting roommates.