As noxryan already pointed out, pilots without commercial licenses can sometimes accept some compensation, namely expense sharing as long as the pilot is paying his fair share of the flight expenses. It's important, though, in those cases that the flight is one that the pilot would be taking whether or not he had passengers.

The problem Flytenow and its pilots ran into is that the FAA decided that participants were operating as common carriers, and flying for a common carrier requires a commercial license.

The FAA defines common carriage as a service meeting four elements: (1) a holding out of a willingness to (2) transport persons or property (3) from place to place (4) for compensation. Elements #1 and #4 are the key elements.

Flytenow argued that its service does not meet either of those. They lost on #4 because expense sharing is compensation. It doesn't matter that it is a form of compensation that is allowed for non-commercial pilots. They lost on #1, I think based on skimming the court's opinion [1], because their service is easily available to the general public.

Note that the FAA does not have a problem with non-commercial pilots sharing expenses with passengers when they are not doing it in a way so readily available to the general public. If you mention to your coworkers that you are renting a Cessna to fly to some specific place to visit family over the holidays and a co-worker says they want to go there too the FAA would have no problem with you taking them with you and having them pay their share of the expenses. Even putting a notice on the bulletin board at work, where it might be seen by dozens of people, that you are taking that trip and asking if anyone wants to come along and share expenses would almost certainly be fine.

[1] https://www.cadc.uscourts.gov/internet/opinions.nsf/77E3D4B7...