They are peers in the execution of 'real work'. What holds them together is a common sense of purpose and a real need to know what each other knows. There are many communities of practice within a single company, and most people belong to more than one of them. John Seely Brown Groups that learn Groups that learn, communities of practice, have special characteristics. They collaborate directly, use one another as sounding boards, teach each other.
Communities of practice are the shop floor of human capital, the place where the stuff gets made.
Not merely peers exchanging ideas around the water cooler, sharing and benefitting from each other's expertise, but colleagues committed to jointly develop better practices.