It usually works in Spain on all airlines BUT normally they only take people from other airlines that have the same procedure.
This was a long battle that the pilots had in their companies, (Iberia as the main contributer) and they got it written in their OM.
So you just have to show up at the airplane with your company ID and licence and ask the captain to go as an Extracrew, and the captain then decides. Your name and ID is written on the loadsheet and then you have full insurence and can go with them.
Normally they know which airlines do the same favor for them... The only thing you have to do in other countrys is fight so they put it in the OM and if all of you do it we can have this privilege all around Europe and be a united pilot group.
E. Mach