From The Art and Popular Culture Encyclopedia
Affinity groups are organized in a non-hierarchical manner, usually using consensus decision making, and are often made up of trusted friends of a common ideology. They provide a method of organization that is flexible and decentralized.
Affinity groups can be based on a common ideology (eg. anarchism), a shared concern for a given issue (eg. anti-nuclear) or a common activity, role or skill (eg. black blocs). Affinity groups may have either open or closed membership, although the latter is far more common.