Nasty Girlfriend´s Friend

My girlfriend has a friend from high school who visits us from time to time. Interesting thing about this girl is that she is instantly talking about sex. She is absolutely open about it, and what brings us to funny situations, for example when we are in a pub and people from other tables start to listen to what we are talking about. I don´t have a problem to tell her anything she wants to know about me and I thing even the shyest person would do so. In the beginning of puberty both girls had kind of romantic early sexual relationship, that´s probably where it all comes from.
Last time she came to see us for several days in the college we had some leisure time in my girlfriend´s room when she told us she had a long term dream to strip in front of some masturbating people and later to follow with her own masturbation or seducing somebody from the audience. It happened to me quite often that I had erections while she was telling some of her stories, but this was really extra. My girl figured out, came to me and put her hand on my bulge what aroused me even more. Then she asked me if I would like to do such a thing with her friend and I told her I didn´t mind if she would be there too. We thought our friend wanted us to do this, but she explained that it must be around four people at least, or so.
My girlfriend knew that I had masturbated several times with the guy from another room in the shower. It mainly happened because we both have a roommates we don´t like very much and there are two showers, so you are always joined there with somebody. So we decided to shower together and eventually masturbated there, but without focusing too much on each other.
So when both girls started pushing me to ask him for this "favor", I finally went to talk to him. It wasn´t a big problem when I told him he could join a private striptease and the only price for that is he could masturbate with the people around. But we needed two more guys. He reminded he had caught one guy in the shower once, but wasn´t sure whether he was masturbating or just had the erection, but since it was a good chance we went to him. Luckily these were two funny guys in one room and when we told them what we wanted they hesitantly agreed.
Next evening my roommate was out so we had my room to do this all. We meet in the dark, we set some lounge music, everybody brought some wine, it was nice evening even without anything. We didn´t know how to start, so we turned some porn on the PC on. I knew my girlfriend has no panties under the skirt, this fact was arousing for me enough, finally I got the idea that our friend could follow some striptease happening on some porn page. This functioned very good, so she started and it was great, she was really beautiful naked. My girl unzipped my pants and pulled out my dick to start it all and she started to do me a hand job while she slightly touched herself under the skirt. When the other guys started to wank I wasn´t sure if our stripper was nervous or aroused, but she finally lay naked in front of us, legs wide spread, masturbating gently but with great intense. I thought the stripper would ask one of the guys to do whatever she wanted. It all followed for a while, I masturbated one more time and we all watched each other cumming, especially the girls who did that as a show for us together. I didn´t know my girlfriend was so much an exhibitionist. Afterwards we stayed in our room just in underwear chatting the whole long night. I would be curious what will happen next time this girl will come, the guys keep asking me whether we could repeat this one day.