Here in Spain it is very usual to travel as Xtra Crew.
This is valid for any crew member (both cockpit & cabin).
You just show up at the gate and tell ground staff that you want to travel on that flight as Xtra Crew. They will ask which airline you work for and if you are cockpit or cabin crew. They will pass this info to the Cptn as he is the one who will authorise you to fly as Xtra Crew. If the flight is full they often take you on a jumpseat.
The only documents you need are your company Id. and your flying license.
Iberia usually just accepts pilots flying as Xtra Crew.
The rest of spanish airlines take both cabin and cockpit Crew.
Normally the captain only accepts crew members flying for spanish airlines, as foreign airlines do not reciprocate on this.