[Theohazard]

Welcome to TFL! I can't quote the applicable law, but I've worked at an FFL/SOT and I've talked to a few ATF agents, and I can tell you that the law is interpreted to mean that it's OK as long as you're in the immediate vicinity. It's fine to let your buddies shoot your NFA items, just stay nearby. As far as leaving them on the shooting bench and walking downrange, you're fine as long as you're not worried about theft. Me, I wouldn't leave my suppressors on my shooting bench unattended, but I don't shoot at a private range like Sharkbite does.