When “expense sharing” trips went from the local airport bulletin board to the Internet the FAA got “uppity”...here is what was happening before the brew-ha-ha: http://cospilot.com/documents/Pilot%20privileges%20and%20limitations_Sharing%20expenses.pdf
and the relevant FAR part:
https://www.law.cornell.edu/cfr/text/14/61.113 (easier to read)
http://www.ecfr.gov/cgi-bin/text-idx?node=14:2.0.1.1.2.5.1.8&rgn=div8 (official govt. site)