I believe 3M makes a door sill protector which is clear and would at least keep your car in original condition when your friends get in and out. Try doing a google search I know I've seen it before. It's clear so no one will notice it except you.
As others have already said this unfortunately is a part of car ownership and it's hard to avoid. The other day I watched a guy get into my MB and he slammed his sneakers into the door sill. I looked......was about to get really pissed:angry and realized I needed to keep my cool. He was smogging the car so pissing him off would be a bad idea. Anyways no damage done this time.
Good luck and let us know if you find the clear protectant.
Tom,
That Magic Eraser is simply amazing. I compare it to a clay bar on the paint, it reaches into crevices and removes unwanted dirt but doesn't harm the original surface. My wife loves this stuff. And no I never thought of trying it on the car, does it easily remove scuffs without damaging paint/plastic?