Just to add to Island Boy's commentary - the regulations do require that the PIC pays no less than a pro rata share. If there are 2 passengers aboard, the PIC must pay for 1/3, for example, no less. There is no requirement for the passengers to pay anything for the flight. That is, as noted, an agreement between all 3 on the flight as to how much the passengers will contribute. In this case, they cannot contribute more than 2/3.