My understanding, in practical terms, is that if a flight is going to take place and will be flown from an advertised departure point to an advertised destination and others go along and pay a share of the actual operating costs, but no more, that is cost sharing.
If a flight or flights are advertised with no defined departure place or destination but the pilot agrees to take someone to a destination of their choice, that immediately falls outside the terms of cost sharing and encroaches into public transport, irrespective of who pays what.
Any form of profit being made over and above the direct cost of any flight, or any money taken by a third party/broker also falls outside cost sharing.