The FAA has a concept of "Common Purpose." You can't share expenses if the pilot and the passengers don't share a common purpose for the flight, that is, there

The FAA has a concept of "Common Purpose." You can't share expenses if the pilot and the passengers don't share a common purpose for the flight, that is, there is some other reason than just splitting the expenses for them to go together, for example, they are all going to the same meeting or on the same vacation. Generally the reason is that the regulations are far more lax for a private operation than a commercial operation, and the FAA wants you to be in a position to judge their character and judgment before you decide to fly under regulations where the FAA isn't guaranteeing much itself.