An affinity group is a group formed around a shared interest or common goal, to which individuals formally or informally belong. Affinity groups are generally precluded from being under the aegis of any governmental agency, and their purposes must be primarily non-commercial. Examples of affinity groups include private social clubs, fraternities, writing or reading circles, hobby clubs, and groups engaged in political activism.
Some affinity groups are organized in a non-hierarchical manner, often using consensus decision making, and are frequently made up of trusted friends. They provide a method of organization that is flexible and decentralized. Other affinity groups may have a hierarchy to provide management of the group's long-term interests, or if the group is large enough to require the delegation of responsibilities to other members or staff.
Affinity groups can be based on a common ideology (e.g., anarchism, pacifism), a shared concern for a given issue (e.g., anti-nuclear, anti-war) or a common activity, role, interest or skill (e.g., legal support, medical aid, black blocs). Affinity groups may have either open or closed membership, although the latter is far more common. Some charge membership dues or expect members to share the cost of the group's expenses.
Although Affinity groups are a natural way for humans to organize and are, in that sense, as old as humanity, the origin of affinity groups in the current context began in the 16th century in Britain with dining clubs that would meet at a set location and at a recurring time. One of the earliest recorded examples of such was a group that called itself the "Fraternity of Sireniacal Gentlemen" which met at the Mermaid Tavern in London the first Friday night of each month. Membership was strictly limited and many of the Elizabethan Era's most prominent literary figures belonged. They gathered to eat, drink and socialize with other members, and to discuss literary matters.
During the 17th century, far more affinity groups formed, ranging from large, national or even international fraternal groups like the Freemasons to private gentlemen's clubs, to small, informal reading circles or collectors clubs. Unlike salons or other periodic gatherings that had continuously changing participants, affinity groups traditionally have had curated memberships. Often, a would-be members must be proposed for membership by an existing member, or would-be members might petition to join on their own. The existing members often are required to vote upon whether or not to accept the applicant as a new member. Some voting procedures require unanimity (such as the Freemasons), others may a simple majority of supporting votes. After becoming a member, continuing membership may be contingent upon conforming to rules or shared ideologies, and the disbarring of members for a variety of reasons is possible.
Affinity groups fall under the category of NGOs, but are further limited by being primarily non-commercial, and are not required to have any specific purpose that might affect the community beyond their own group. For example, both a social justice group and a philately group would be considered affinity groups, but the former more closely resembles the classic definition of an NGO.
Political affinity groups 
Affinity groups engaged in political activism date to 19th century Spain. It was a favourite way of organization by Spanish anarchists (grupos de afinidad), and had their base in the tertulias or in the local groups.
Politically oriented affinity groups in the United States gained public attention during the anti-Vietnam War movement of the 1960s and 1970s. The term was used by Ben Morea and the group Black Mask. Later, anti-war activists on college campuses organized around their interests or backgrounds -- religious, gender, ethnic group, etc. They became popular in the 1970s in the anti-nuclear movement in the United States and Europe. The 30,000 person occupation and blockade of the Ruhr nuclear power station in Germany in 1969 was organized on the Affinity group model.  Today, the structure is used by many different activists: animal rights, environmental, anti-war, and anti-globalization, to name some examples.
By definition, Affinity groups are autonomous from any larger body. Co-ordinated effort and co-operation amongst several Affinity groups, however, is often achieved by using a loose form of confederation. Private clubs, for example, may cooperate through reciprocal agreements which allow the members of one club to use the facilities of another club in a different location. Other affinity groups, such as Rotary or Toastmasters, may be individual units that conform to shared standards so that one may participate in another group of the same name anywhere on earth without requiring the individual to reapply for a new membership.
- Cluster: The cluster is the basic unit of organization amongst Affinity groups. A cluster consists of several Affinity groups and is organized in a non-hierarchical manner. A cluster can be permanent, but is more often an ad hoc grouping organized for one specific task or action. One can be organized around a shared goal (e.g. blockading a particular road), a common ideology (e.g. the Quakers) or a place of origin.
- Spokescouncil: The spokescouncil is an aggregate of clusters and Affinity groups. Each Affinity group or cluster nominates one representative (often called a "spoke") to participate in the council. Spokescouncils are most often temporary bodies, committed to accomplishing one task or event.
Affinity groups tend to be loosely organized, however there are some formal roles or positions that commonly occur. A given Affinity group may have all, some or none of these positions. They may be permanent or temporary and the group may opt to take turns in these roles, or assign one role to one person.
- Spoke: The individual charged with representing the Affinity group at a spokescouncil or cluster meeting; roughly the same as a spokesman but without gender assumptions. Occasionally, the spoke will be granted a more general ambassadorial role by the Affinity group.
- Facilitator: A person or people who perform facilitation duties in consensus process of the group and also, to varying degrees, act as arbiter of internal conflicts.
- Media contact: An individual who represents the group to the mass media. Often this individual is the same person as the Spoke.
- Vibe watch: A person or people charged with monitoring the mood and feeling of the group. The reference is to vibrations in the colloquial emotional sense. In some Affinity groups, the vibe watch is also charged with keeping the facilitator from using his or her role to favor any position or proposal.
- Snap-decision facilitator: Also called "quick decision facilitator", this is a person charged with making decisions for the group in time-constrained or high-pressure situations. The position is rare and is almost always temporary (contrast with the pre-Imperial Roman concept of a temporary dictator).
- Hauser, Luke Direct Action: An Historical Novel, (New York: GroundWork, 2003) 768pp. ISBN 0-9740194-0-2
- "Collateral Damage" Civil Resistance Action Against the War on Iraq, Legal Defense[dead link] Colorado Communities for Justice and Peace, July 29, 2003
- Rant Collective : History of Affinity groups
- Starhawk.org : Affinity groups
- Uproot : Affinity groups
- Seattle prepares for battle - Trade before freedom
- Rant Collective : Clusters and Spokes Councils
- Austinspokes.org What is a spokescouncil?