As a car owner and observer of people, for many years now, some of this can be corrected, some won't. I can use a metaphor. I
My girlfriend and I bought a house together a few months ago. Although
it wasn't too expensive, it is in a good condition, we take care of
it, and like it very much.
We have a lot of friends and a busy social life. We often have people
over to our house as they all have apartments and such that are too
small to accommodate the gatherings. Unfortunately, some of them
simply do not respect our home as we do. They have already made minor
scratches on the counter, stains in the carpets and visibly make
themselves at home. I would like to stop it once and for all.
Here is the issue where I say some can't be stopped. The more people you load into a car, the more it's at capacity, the more likely things will get damaged. There is also the issue that you are loading a bunch of men (likely young?) with the energy of just coming off a sporting event to travel to another location enclosed in relatively tight space, and likely their behavior will be a bit exuberant.
So I would say there are likely two things here. You can for sure try to minimize the wear and tear on the car from giving these rides, but you won't eliminate it. The more people you put into any space, the likelihood of damage goes up exponentially. The more energetic/pumped up/exuberant the passengers, again, another risk factor as people are distracted and overly rough with their bodies when excited.
So I would simply say it before loading in the car. As I walk toward the car with the people I am going to transport I'd say something like
Hey guys, please be a little careful getting in & out with your gear.
There have been a few accidental dings and scratches and I might want
to resell this thing. You all are awesome, but transporting 5 sweaty
athletes isn't likely going to ever be a selling point.
And expect some damage. You really have to. It's part of life. We have hosted lots of parties. When we first bought a house I used to find it so frustrating how impossible it seemed to keep such issues at bay. Eventually I realized you can't. It's part of agreeing to host (I don't mean horrible stuff, but stuff, like a scratch in your wood floor) and that I was either going to need to accept that as part of the deal or stop hosting. I like people and parties too much to choose to stop, so instead I chose to stop sweating the small things and enjoy it (and fix as much as I could after & call the rest "home character").
My car also shows plenty of signs of all the people that have ridden in it. I have kids, I had this same car with older kids, I have transported adults, babies, kids, teens, mostly my own, but also plenty of others. It's going to show. You have to adjust what you expect so you aren't being unduly stressed by things you cannot control. AND just to be clear I don't even allow food or drink other than water in my car, and still, it just shows wear. That isn't because we don't take care of it, it's just because lots of people have been through it.