Stormi is correct there is an option to fill empty houses but unfortunately it is currently a known bug and is not working. You can however manage households from the manage world's area and be able to place homeless families in homes.
I've seen sims move into the starter level houses, i actually got stuck in my game because someone that had moved into the house next to the Glade tree caught himself on fire and died. I had to wait until the fire burned away everything in the house before I could leave.
The larger houses, the "family" seems to remain, ie: Goth, Landgraab; even tho all the sims in it have passed away... not sure if the homeless will move into them tho because of the cost.
But however it's populating wasn't fast enough for me, I ended up filling them all myself and turning their aging off.