People try that on here, @Heather1244 , and my strong advice is the same as everyone's above -- say no -- and use that "only registered and paid guests" and "not exceeding capacity" wording @Anonymous recommends.
I regret that one time, when I was a new host, that I let an extended family come with tents. They were a ton of work and although I hadn't charged them extra they knocked stars off value and overall because "it wasn't as private as we hoped."
No kidding. Of course a private place isn't as private as you hope when you bring your mother-in-law with you!
But that pales next to what is probably really going on with your request -- it does look like a party wanting to happen.
@Sarah977 has a good point about the bedding, too. If it is the registered guests who want to sleep outside "for fun", they need to do that in their own sleeping bags, not on your bedlinens.
Nothing about this sounds good, and it seems you sensed that, too, since you are asking.
I've had to say no to this quite a lot lately, and I always say at the end, "I understand this is a deal-breaker, and no worries at all if you need to cancel and find someplace that can accommodate your group."