Make their wishes come true. you will see a box next to the picture of the person and 4 boxes underneath it, make the thing in the top box come true and their lifetime points will go up :)
Go to the status panel so that tou can see all the progress, info, and stuff. Then click LB until you get over to sim info. Click right on the D-pad and if done right you should be there. :)
Actually, u cant be a girlfriend/boyfrind with 10 sims at once but u can break up with them and go with another sim and break up with them till eventuall u have dated 10.