Quite a few guys. I'll still have some morning wood after showering in my dorm and I towel off in the open, so some of the guys in my hall have walked in on me hard (some other guys also towel off outside the shower, but they're usually semihard at most).
I slept naked at hostels, and if it was late and thought no one would be up, I'd walk naked and hard to the bathroom. Of course, sometimes it turned out there was someone in there or walking out that I had to pass by. I would just casually say hi and go on. One guy complimented my boner as a joke.
I was changing at the gym the other day and a friend of mine passed by while I was naked. I greeted him (I was still naked, he was clothed) and we chatted for a bit. While having a casual conversation, I started getting hard and couldn't grab my towel without making it obvious, so I nonchalantly grabbed the head of my dick as pushed it down as it started growing. Our conversation was actually really long (like 10 minutes) about classes and exams etc. The shaft of my dick was clearly fully hard, and my palm was filling with precum. He pretended not to notice. At the end he offered to shake my hand, but my palm was literally slick with precum, so I had to turn him down saying I was sweaty and my hand was dirty...