It only comes up under the friendly interactions and you have to only do friendly interactions for a while to get it. They have to be "sociable" with each other and you need to get a lot of the other "Asks" out of the way, example Ask about Day, Ask about Book, Ask about Career.
Don't interrupt with a funny or romantic interaction because that means you have to start all over.
Also you can't move in someone if your household is full. You can only have 8 Sims in a household including roommates. You can have up to 6 pets as well but only a combination of up to 10 so if you have 6 Sims and 4 pets. No one can move in.