In the middle of the high school I went with a bunch of friends to the mountains for about ten days of hiking. The only place to stay (cheap and in a good location) we found was a new build family house with a landlord family living in a small old house temporarily attached to the new one. They were connected, so they could control the whole place. It has two floors with some living room and restrooms downstairs and four rooms upstairs, but it was all empty, only one room was equipped with one big sleeping place. The problem we didn´t mind was there were no doors yet, they were just waiting for delivery of them, that´s what we were told.
During the days we were off and after getting back quite late we were tired enough to get to sleep, so we didn´t mind any kind of uncomfort. Bigger issue was the fact there was not enough hot water for everybody´s shower, some people had always to wait until the heater warmed some new water. So we decided that each night the different two of us would wake up in the middle of the night, would use a shower and then water will be ready again in the morning. We were eight people, so for everybody it was two interrupted nights only.
We always went to shower two at the same time, while one was having a shower, the second one was standing in the door place, watching if somebody from the owners family was coming. There was only a shallow bathtube, so we saw each other naked, and there were still some other guys from our group walking around, so after a while we didn´t mind being seen.
One night the other guys got up at 2am to have a shower and they woke me up accidentally. After a while I decided to go to the toilet and get myself some drink. While walking downstairs I heard one guy asking another whether he was already finished. I didn´t find anything strange in it so I just went there. When this guy in the door saw me he only whispered word attention to the other one and when I looked inside I saw him hiding his obvious erection. I was a bit surprised, so I only told them that it was a good idea, thinking I also would like to masturbate. When I was walking back they changed in guarding and the guy who was masturbating before asked me not to tell anybody. I told him I was sure everybody wanted to do it so even if they would all know it wouldn´t mean a thing. I asked him if they did this all the time, he answered me no, but the other guy from inside started to laugh, so the answer was different.
Since this moment it somehow didn´t go out of my mind. So the next night, when it was my turn to have a shower during the dark, while he was already in the bathtub I told my mate what I saw the night before and offered this to him. He told me he wanted to masturbate but would do it only after me - he was afraid of it might be some kind of game I was playing. So he only finished his shower. When I got in I decided to do it as he obviously didn´t mind. So I told him not to watch me, if possible. First it was pretty difficult intentionally to get the erection, but once I had one, I wouldn´t mind even if all of us guys would see me, I was so horny after those days. He turned once or twice to ensure I wasn´t kidding and after I had a quick and strong ejaculation and stopped the water he asked me if I would guard the door while he would do it inside. Of course I said yes, so he went there and masturbated quickly while standing back to me.
After this we did it every time while showering. We had some certain signals because during the day everybody was around. But with some skills we could do it even then. The two other guys I caught the other night found out what we were doing one morning and we started to joke about it, so finally the other guys knew it too. Some were surprised, but mainly they all thought how to tell to their mates they wanted it too.