National Aviation Authorities & Security Regulations should always be adhered to Airline policy on admission to flight deck is: No person, other than flight crew members assigned to a flight, shall be admitted to, or carried on, the flight deck unless the commander himself or his designated deputy is present on the flight deck.
The final decision as to the disposition of other vacant crew seats, again, rests solely with the commander.
Normally, such permission will only be granted to employees of the operator, or of other operators, or to employees of aeronautical or other authorities.
(e.g., crew seats which are not certificated for occupation by crew members during takeoff/ landing or specified other phases of flight e.g., because of emergency – evacuation problems, shall not be occupied by other persons).
The commander shall not grant permission for occupation of a vacant crew seat located at an emergency exit to any person who has be denied seating at such exit or to any other person lacking sufficient strength or dexterity to operate and open the emergency exit, to exit expeditiously, and to assist others in getting off an escape slide.
The crew member responsible for safety in the cabin shall brief a person who has been granted permission to occupy a vacant crew seat on all safety relevant aspects connected with that seat and, if it is at an emergency exit, on how to operate and open the door in an emergency (stressing, however, that the door shall be opened only after the appropriate command has been given, and the crew member assigned to that exit is not capable of performing these actions).