I want to tell my girlfriend that she needs more friends
Why do you think she needs more friends?
I want her to meet new people and not be so lonely.
If you think that your girlfriend is unhappy, then you should try to help her make new friends. However, you shouldn't decide how many friends she should have, or pressure her into doing things just because you think they are good.
I know she wants some friends. She's not even on Facebook.
Well if she has mentioned that she wants more friends then it's good that you want to help her with that. Maybe you can get her on Facebook as a start.