Along with anyone else who might be entitled to a ticket I guess, given 500000 applied. I presume they just work their way through the priority groups until they're sold out.
The record was set a number of years ago (me thinks someones needs to brush up on their German football knowledge and how big a club Dortmund are) They are the trail blazers of German european football.
A bit harsh. They have a massive local fan base and hold a record for the most people in a single place watching a game (or something similar) when they played a title winning game at their ground and had a big screen also showing the game in an adjacent park.