Note: The FAA has recently responded to a letter from Airpooler.com with the opinion that carrying passengers in common carriage for compensation is prohibited by FAA regulations. The opinion makes no mention as to whether a pilot may fly with another pilot and share the cost of the flight. Note #5 on page 3 of the opinion letter does seem to allow it as long as the pilots share a common purpose. However, out of an abundance of caution, the author of this website is drafting a letter to the FAA for clarification of this point. Until that clarification is received the only option available for flight cost sharing in the listings will be "not required."
To read the letter yourself: Click Here