My quick and dirty summary.... and sure not complete, IANAL etc.
Ignoring medical transport and special airworthiness etc
Cost sharing doesn't apply for this as it's more than 6 seats fitted.
Cessna Caravan operated by Farmer Joe for a flight to town to get on the sauce.
Not Passenger transport if:
- Farmer Joe covers all the costs of the flight and his mates don't pay or reward him in any way (Part 91)
- Farmer Joe is on board alone (obvious) (Part 91)
Passenger Transport Operation if
- Joe's mates cover the cost of the flight (Part 135)
- Joe covers the cost of the flight but his mates pay for any item of food, drinks, accommodation, or offer him e.g. sexual favours etc (Part 135)
- Farmer Joe is on board and all the other pax are given a free lift into town and do not pay in any way (Part 91)
Same Cessna Caravan operated by Joe's Farm Company.
ALWAYS a passenger transport operation
- If no passengers pay for the ride in any way in money or Joe receives no reward from them - (Part 91)
- If they are his employees and he (or his pilot) is getting them from town and picking them up* (Part 135)
*this is in the AGM, and an interesting one as I'm not sure what the "reward" is there for the employee?