Since those NPC's that they bring home are teens, you'll never be able to have them move in with you, but if you go to the Neighborhood screen and go to where the Sim bin is, you'll see an option that says 'Send Sims to college' under that menu is all of the teen Sim NPC's. Send them to college and play them there until they Graduate or once at college just have them drop out to send them back to the Neighborhood and in the Sim bin where you can have them move into a lot and play them there.