Oh good grief... I had to read a recent FAA subscription mailing.
A major point of emphasis to keep in mind regarding expense sharing flights is the “common purpose test”. This means, that the pilot must have his or her own reason for traveling to the destination, not simply for the transportation of the passengers.
For example: A private pilot is flying to Stillwater, Oklahoma, to visit her mother in the hospital over the weekend. Five of her friends would be coming with her to attend a football game that same weekend. She CAN legally share expenses because she has a reason to fly to Stillwater (visit her mother) not simply to transport her friends. Expanding the same scenario; if she has too many friends going to the football game that she has to make a second trip to pick up the rest, she CANNOT legally share expenses on the second trip because her purpose for flying to Stillwater was complete when she arrived the first time. The second flight was solely for the transportation of passengers.
And what about the return trip? I have to really want to go back home?
I don't remember things like this being an issue before, maybe they've changed the rules over time. And they are carrying it over into drone operations too.