I've seen this show long time ago, which I kind of annoyed with the staff as I saw hoshi sweating a lot during the show, and some of the member also. The lighting must be too strong so the the temperzture of the room must be too hot for them to enjoy.
Which show? If it's the first show, that's not sweat, but water. You can watch it in 4.35, there were water leaking from the tool that wonwoo wore in his forehead. All members wore that tool.