The Genei Ryodan (translated as the Phantom Brigade, called the Spider by those who know a lot about them) 13 members. They each wear a tattoo of a spider with 12 legs (counting the head, it makes 13) with the number of entrance into the clan tattooed in the middle. The members can change at any time. The ones who want to enter the Ryodan must fight against a member to replace him. A part from this, if some people are missing, the head can choose to integrate new members without fighting.
Their main activity is murder and robbery, but sometimes they act for the good (of who is not said, but presumably the Ryuuseigai).
The Ryodan's organizational structure is rather lax. Members meet rarely, before York Shin they hadn't all met for three years and sometimes some members supposedly don't show up, or are not invited, or the group splits into smaller teams. Kuroro Lucifer is the "head" (of the spider) and is called Danchou (leader) by his followers. Although his commands come first and he controls the members, his life (as the life of the leader) is not their top priority.
The Ryodan is rare in that they, unlike many organizations, can keep collected
even without their leader. Although the spider loses its head, it continues
to move.
There don't seem to be many rules within the Ryodan. The members are forbidden
to seriously fight each other (although scuffles are common). Arguments that
cannot be overcome are solved with a coin toss.