Imagine throwing a pebble into a pond and watching the concentric rings expand out from the splash. This visual is the key to the concept of concentric circle recruitment, which is comparable to, say, enlisting your friend as your band's drummer as opposed to holding open auditions.
The central idea follows a basic line of reasoning: Who better to recruit as a volunteer than people who are already connected to you via some other relation. All an organizer has to do is move out in concentric circles through an existing framework of family, friends, coworkers and acquaintances.
If you need a volunteer to build your organization's Web page, then hey, why not recruit your brother's girlfriend? She's a freelance Web designer. Need someone to help distribute flyers? How about some people from your bridge club? Perhaps the most basic example would be a band turning to existing fan club members to help with an upcoming gig. Essentially, they're already recruited; they just need to be organized into a street team. Concentric circle recruitment then spreads out through the expanding web of connections that grows with each new volunteer: their family, their friends, their coworkers. Recruitment becomes self-sustaining.
Concentric circle recruitment tends to prove effective, thanks to personalized appeals to individuals who are already in close contact with the organization's volunteers, if not their actual volunteer work. Some may even be former recipients, such as a recovering addict who serves as a volunteer sponsor in an addiction program. The downside is that, since concentric circle recruitment tends to move through existing groups, it can lack the diversity and "fresh blood" that wider forms of recruitment attract.
The fourth recruitment method, ambient recruitment, generally depends on geographic or organizational communities, such as a neighborhood or school. The idea is to promote a community atmosphere that values volunteerism. Community-wide "help us keep our neighborhood clean" campaigns are an example of this, as are the volunteer efforts leveled by individual churches, schools and places of work.