If you want to play with apartments and have sims living in each one, you can use the multi owner doors by Flyby at MTS. These doors can be set to only allow the owners inside a particular flat. Visitors arriving at the lot can be given keys to enter certain doors. The keys disappear again, once the visitor leaves. I just tested them in my crayola apartment complex, and they worked fine. The only downside is that it’s only a number of doors that’ve been modded so there’s not a lot of choice.
Thanks to Sandy for making me aware of this set!