Instructions
Complete the missions until you have gotten rid of the ghosts in your house. If you haven't been doing the missions, you must begin now and continue them until the ghosts have been removed from your Sims' home.
Find a Sim who doesn't have a place to stay (you'll find them wandering around. They never go into a home and stay there for a lengthy period of time). Begin talking with her and building a relationship with her until you have a level four relationship and a level four love.
Return to your house and walk over to your phone. Use the phone by pressing the "O" button.
Select "Call a Sim" option to open a pop-up screen with a list of Sims.
Select the Sim that you want to have as your roommate and press the "O" button to invite her over to your house. Answer the door shortly thereafter to allow her in your home.
Walk over to her and press the "X" button to greet her. A list of options will appear in the upper right corner of your screen. Choose the "Invite" option, followed by "Invite Roommate." If she accepts, your Sim will now have a new roommate.