How do you get rid of an annoying girl?

So I have a group of friends, and we're pretty good friends. There's about ten of us. So one day, this girl that goes to out school starts practically stalking one of my friends, let's say his name is John. So we were all talking one day during break, and this stalker girl practically invites herself in and starts telling us about her 'mental instability' and how she wakes up in the middle of the night. I was like wtf? Everytime one of my friends and I are talking, she'll invite herself in and bring all attention to her and it really irritates me because she just came out of nowhere and just decided that she was going to follow us. I would normally have no problem with this because; I'll be honest here; our group is kinda perverted. Not to an extreme, but a few "That's what she said" jokes here and there. But with her, she gets offended by every little thing and practically sucks the fun out of everything by turning the conversation to her stupid problems. I don't know what to do now because it's IMPOSSIBLE to get rid of her. >:/ (Wow, I just needed to vent) (P.S.S: The group consists of about 5 girls and 5 boys EXCLUDING THE STALKER.)