This happened when I was 14 and my sister was 16.
Our parents went away for the weekend and left my sister and I at home. My sister asked my parents if she could have some girl friends stay the night. Our parents didn't have a problem with that. It turns out my sister had six friends over for the night. I didn't have anything going on so I stayed in my room and let the girls have the run of the house. I was laying in my bed when suddenly the door flew open and in come 4 of my sister's friends. They came over, grabbed a hold of me and carried me down to the family room, I knew I was in trouble! They set me down on the floor and ordered me to take my clothes off. I refused knowing full well they wouldn't stand for that. I had a pretty good idea what was going to happen. I didn't have a problem with that but I wanted to be 'non-cooperative' anyway. Next thing I know they start holding me down and stripping me. Now I'm laying on the floor, nude. They now ordered me to masturbate until I cum. I told them that was impossible (yah, right) since I can't cum if they are going to watch me with their clothes on. To my surprise I was suddenly being watched by 7 naked females. I started playing with myself while they watched and had the first of many orgasms that would occur that night. There was no intercourse that night but there was a lot of masturbating both alone and mutual.