My first masturbation experience with others involved my best friend, Lily, and her brother, Colin.
It was a really hot day and we were playing in the garden, behind the shed. Colin and I were talking about sex, as boys of a certain age are wont to do, when Lily came and joined us. She was wearing a light summer dress and no panties and then, for no reason whatsoever, she simply lifted her dress and showed us her beautiful white pussy.
It was the most amazing sight ever. I hadn't really seen a girl's bits up close before and it was really pretty - the cleft, the folds of flesh, the silky whiteness of her naked body, and the lovely swirls of fresh pubic hair. She didn't shave yet, like the other girls at school, and she clearly had an exhibitionist streak.
I felt a familiar stirring in my undies. (Colin and I had been sunning ourselves in our undies, having stripped down earlier in the afternoon). Soon, a bulge was clearly visible in my pants and a small stain of pre-cum appeared.
Colin also got aroused, watching me get erected, and stuck a playful hand down the front of his y-fronts. Then, playfully, he stuck a hand down the front of my undies, in full view of his sister. Pre-ejaculate flowed everywhere. My dick was just swimming in moisture, especially as Colin slid his hand up and down my shaft and liberated my cock from my briefs.
He stroked my cock for about half a minute and I sat back and relaxed, enjoying the sensations. Finally, in a gush, I came, after telling Colin I was "very close". He stepped to the side and watched in awe as white goo shot forth from my penis. It was the first time I'd come from a hand-job. Before that, I didn't even know if it was theoretically possible.
Lily was sitting across from us the whole time, watching with a queer expression of fascinated pleasure on her face. Occasionally, she touched herself down there and fingered herself. When I came, she seemed to get embarrassed or to remember herself. Making some excuse, she jumped up and ran back to the house.
Colin and I finished off the afternoon, sun-baking naked, and kissing. I even licked Colin's sexy brown cock and he rubbed himself vigorously so he came too.
Often after that, at night in my bed, I fantasized about the sight of Lily's beautiful exposed fanny and what thoughts might have been running through her head ... She was a lovely young woman, and her brother was pretty cute too.