Either marry the Sim or use the "Ask to Move In" action on them when they're on your home lot. The relationship level needs to be high to perform either of these actions.
Chat with our AI personalities
You go to there inventory and click on the phone. Then you click on the move option.
all you have to do is if you live with them make one relax on the bed then switch person and get them to join them if they dont live together get them to marry and they will move in together
You have to go to a computer, phone, or newspaper and then click the "move" option.
Sims have to go potty.
when they move out, they will enter The Clipboard (the place where the Sims are kept after you've made them but before you move them into a house. From there, you can move them into a new lot to play them.