Structure of a release group

Organization- Leader
The leader of the group is the "boss". He decides the main directions for the group. The leader is not a dictator, he won't decide everything by himself. He also has to keep the group together, and keep the individual members satisfied.
Supplier
The supplier is the group's source. He often has pre access to the game/movie/etc, but this is not necessary. It's also possible it's someone who sneaks into the cinema and films the movie. The top groups have pre access. Their supplier might work at a dvd plant, a dvd review magazine, or a dvd rental store. The supplier gives the game/movie/etc to the other teammembers.
Cracker
The cracker breaks the security. Not all groups have a cracker. Crackers are required to release games, applications and alike.
Encoder
The encoder rips and converts the movie so it's suitable for the web. Encoders are just in movies/dvdr groups.
Packager
The packager packs the release and adds the essential files and information.
Courier
The couriour pre's and spreads the release all over the world via FTP.

This is just a global overview, it's not the same for every group. Music releases for example are often quite simple to create and multiple tasks can be done by 1 person.