Let's take a sharing example. You want to build up cross country time, so you let folks know that you will take them on business trips to different places and will charge them only a pro rata share of the flight operating costs. You could care less where those locations are; you just want the partially-paid flight time. Looks a lot like an Air Taxi giving discounts, huh? Chances are it will to the FAA also. What the FAA requires for sharing expenses is, at a minimum, that the destination is someplace the pilot wanted to go, regardless of whether he had passengers or not. There will be a number of ways of looking at that choice, but an easy start is, whose idea was the location?
I tried to do something similar in high school. I put flyers in the school office, even had a breakdown of the costs. I was going to do pro rata flights for dates, proms, etc; take your honey flying, pay part of the cost, I'll pay the rest, we'll go look at the lights, etc.
The local flying club got wind of that scheme and next thing I was in the owners office for an ass chewing that would put a doberman to shame. He got hold of me before the FAA did.
No more flyers.