Aside the legalities- My advice is I think it's worth having someone neutral on hand who isn't joining in to keep an eye on things, answer the door, take any calls- people will want directions, be late, confirm and so on. They can also serve drinks, hang coats up, keep things moving and also making sure they are using condoms- It's much harder to keep a track on such things if you've got a cock in your mouth and numerous men around you.
It's also useful if you need a break to have someone saying can you give her 5 minutes and if you are in a flat an extra person can keep an eye on people not wandering into rooms they shouldn't be or being naughty in other ways. That way you can focus on the job at hand.