Well, it's been a long time since I've been involved in a sleepover, but when I did there wasn't usually a problem with taking off our clothes in front of each other (all guys there), at least in situations where the parents of the house weren't in the same room.
Sometimes we did skinnydipping at sleepovers if it was at a house that had a pool, if we knew that nobody else minded doing so (otherwise we went in swimsuits, and if someone wanted to change alone in the bathroom, nobody else held it against them). Surprisingly, a lot of us did sleep in just our underwear at sleepovers (though again, if someone wanted to sleep in pajamas/sweats and wanted to change into those privately, they were fully welcome to do so and nobody held it against them, though it didn't happen too much as we got older).