A Circle is a concept which is distinct from a group.

Groups are defined by the what, a constituent number of people who get together. 

Circles are defined by the how, the manner of being that that people take when they get together. 

Groups come together and have a leader; someone who decides how things will progress and guides the others—the participants—through the process. 

Circles have an initiator, someone who calls individuals into a special communion with each other and leads only until the members decide together how to proceed.

Groups come together in the mundane world to do important work. 

Circles call you into a sacred space in which to do sacred work.