My pumpkin parties are BYOP, but I've always provided all the tools and patterns. That's easier to do when you're the Patch Master. I have a toolbox full of scoops and saws. Not to mention several hundred ZP Pro Carving Tools.
My guests did the gutting at the party, but with the Pumpkin Gutter on the drill, it went pretty quickly. I had large bowls/buckets/pots on the center of the table, which we'd periodically dump into a nearby trash can.
I have copies of all my patterns in plastic sleeve pages in a couple 3-ring binders. So I let my guests flip through those and pick what they wanted to carve. If you have a computer nearby, they could browse/search this site to pick their patterns.
I supplied the transfer paper, and offered instruction to those who needed it. I had cups full of tools in the middle of the table too, so people could grab whatever they wanted. It got messy, but thats what the plastic table cloth is for. At the end of the night, just roll up all the junk into the plastic like a giant garbage bag (make sure you pick up all your carving tools first, of course!).
For you, maybe you don't have as many supplies on hand, so I guess you'd have to ask them to bring what they'll need with them. Gutting the pumpkin before they arrive is a smart idea, if you're concerned about mess and the amount of time you have. The more steps you have them do before the party, the more extra time you'll all have to mingle after you carve your masterpieces.
I've done some community carving events for the local kids, and the rule for those is to always ask the kids and parents to arrive with their pumpkins already cleaned out. But when I see what some of them show up with, I sort of wish I could have shown them how to do it properly.
It really puts all my skills to the test, when a 3-year-old demands that I somehow make the Spider-Man pattern work on a 6 inch tall pumpkin that is still filled with seeds.