The club has about 44,000 members and they are the owners, so it's a fan owned club as i think Barca and Real Madrid are but the set up in Bilbao is more for local fans the other 2 are more interested in "rich" members rather than local? Then to play for the club you have either got to be born in the Basque region of Spain or come through the ranks of the club from junior level, they don't sign anyone who doesn't fit the criteria, basically everyone who plays for the club grew up supporting the club.
Only 1 of 3 clubs in Spain who have never been relegated from the top division and the 4th most successful club in Spain, apparently founded by the English! Dockers and sailors about 140 years ago bought the game to Bilbao and it went from there.
Will now be my club of choice in Spain, not that i really support anyone else but West Ham