As a former Customer Service agent for a major airline I can tell you that you MUST be safety trained to sit in the cockpit or cabin jumpseat. FAA also can do it as stated above. Many airline pilots and FA commute and are able to get a jumpseat when no passenger seats are available.
Personally, back in the 80's when things were a bit more relaxed I got to sit in the cockpit with a Capt. who was soon to be retired. VERY cool experience, especially landing in SEA in crosswinds in a 747. Ohhhhhhh yaaaaaaaaaaa, those were the days.......