How exactly do you invite a roommate on Sims 2 for psp?

After you have cleared the ghosts out of your house and found someone who is always walking around with no place to stay(but make sure the sim your gonna invite cannot talk to you).They could be in deadtree, paradise place or division 47.Get your friendship and romance to level 4 with the sim,then go to your house and use your cellphone to invite that sim over. once they are over at your house...push the x button next to them and it will say invite roommate. =D that's all.