Once the captain has signed for the aircraft then they are in command of it and thus anyone actually on that aircraft are required to follow their commands (if legal). There is no mention of the doors needing to be shut.
Possibly 747Jock - but the captain can't sign the load sheet until all pax are seated and the next and final item is the closing of the doors. When the captain has signed both the Tech Log and the Loadsheet he has accepted the aircraft.