It's difficult to choose the category. This happened on a Saturday when I was 15, and was home alone knowing that my parents and sister would be away for the entire day. I planned a day of leisurely jacking without interruption.
My parents and sister went to visit relatives who lived out of town and would be away for at least 12 hours. When they left, I went to my room, stripped naked, lubed my peter, turned the TV to a porn station, and began to jack. I was a good edger, so I made the first session last about 30 minutes, and had a come of about five squirts. I lay on my bed watching two men in the porn movie working on one woman -- one of the men was sucking her tits while the other had his penis half way into her pussy. I was hard again, and began jacking my 6.5 incher when I heard an unexpected sound behind me. I glanced over my shoulder, and there was Sue my sister's best friend looking at me wide eyed. I immediately covered up with my comming hand towel and began apologizing. She smiled and said not to be embarrassed that she and my sister jilled together, and each of them masturbated every day. She asked if she could join in with me. I agreed; she went to my bathroom, took a pee, and came back naked. She lay down on the bed next to me, took my peter in her hand, and gave me a wonderful hand job. This come was faster than the first.
While we were relaxing, she said, 'You and Sam (her brother) jack off together, don't deny it. I know what you do when you come over to study.'
How do you know that, I asked?
Pam (my sister) and I can see into Sam's room from my closet. We usually come together with you and Sam when you two are jacking in his room.
How did you get in? I thought I had all the doors locked.
You didn't lock the side door. When Pam told me that you weren't going with them, I knew that you would be jacking off all day, and I wanted to watch.
How could you know that I would be jacking off?
Pam knows you jack off every day. She can hear your moaning when you come and the slurping noise of the lube you use. Sam does the same thing; you two must share experiences.
All of this was true. Next she suggested that she and her brother and Pam and I get together for fun and games. She said that she would suggest it to Pam, and I should suggest it to Sam.
The next day, I told Sam of Sue's suggestion. He had already been talking to Sue, and readily agreed. That left Pam, but she had been talking to Sue as well, and was anxious to start the new fun. All of us were ready. I may add to this story.