Well, first I'll say this: Nobody comes to my shop unless I invite them first, and so far it's mainly been very close friends and family. Now I have invited a handful of new or budding knife makers as well, and a couple of them have accepted the invite once or twice. Occasionally I'll hand out some materials help a guy get started, or make some small parts for a close friend, but I don't do anything I don't want to do, and it's never with an expectation of a returned favor. I do it because they're my friends or family, or because I want to give another maker as much help and I would have liked to have had starting out.
I have found that your true friends with volunteer or try to return the favor without being asked, though I usually protest.
Now, if it got to a point where I was being asked to use my shop, tools, materials, etc... and it had any kind of a chance to become a semi-regular occurrence, I'd lay out the ground rules immediately.
Buy whatever blade steel, belts, and other consumables you think you'll need, and you can use my machines (under my guidance/supervision) to make it happen. Bring a case of beer and I'll order the pizza.

I expect most people understand that nothing is free, and things cost money. If they can't understand that, then they likely don't have the capacity to run a power tool either, if you catch my drift.