I fly for Southwest Airlines. You have to be working for a airline and your company has to be on our jumpseat list. The FAA, Inspectors and Air Traffic Control, can ride also. We would love to have foriegn carriers but our FAA will not allow that. When I was with Pan Am we had foriegn carriers on our list.
On the ground you are welcome to come up to the cockpit. We lock the door from takeoff to landing.