I know that people are always hungry for a free lunch. I work much harder than most people to have the things I have, yet they all wants to borrow my toys and my tools. They complain about how they are less fortunate when I know they are really just less motivated. I work 60+ hours a week at work or school, a number of people I know who think I'm stingy sometimes spend 60+ hours a week playing video games. Greedy lazy people have found that they can take advantage of hard working people by accusing those people of being selfish. Well, I don't bite.
I only let people I know ride my skis. I only let people at the lake that I have ridden with many times to ride my ski. I only let strangers ride my ski when we are trading ride time (My SJ for their SXR, B1, etc). All of my friends know the 1st ride is free, and after that, they need to chip in on gas, after a few rides of paying gas money, they need to be buying their own stuff. I will let people try my stuff if they legitimately want to get into the sport with money in hand (my ski not for sale but I didn't have the opportunity to try before I got into the sport so I like to make that available to others.) There is of course the exceptions of people who do me favors, and ride time is one way I use to repay that, but those are obviously not the free-ride kind of people.
Call me cynical but most people only care when the ride is free, and they don't care if they break your




. For years I've let people ride my quads, got no gas money, specifically said "Don't take it in the mud" to spend the next day pressure washing 2" of mud and replacing brake pads on a quad I didn't ride. No help, no money, just being taken advantage of.
The answer is "Sorry, I don't let other people ride my skis".
You should also tell your friend that invited his 7 friends that he should not abuse the friendship, and feel comfortable telling those 7 people you didn't invite them and are not comfortable with the situation. Anyone who can't comprehend your situation, should not be on your ski anyway, because they don't appriciate the cost, the maintenance work or the liability you are facing. If they can't comprehend it, why should you trust them.
It's easy... Just say no.