Ny social network that is defined by a common purpose and that has a boundary between its members and the rest of the social world is known within sociology as a (n) :

Or·gan·i·za·tion ˌôrɡənəˈzāSH (ə) n / noun noun: organization; plural noun: organizations; noun: organisation; plural noun: organisations 1. an organized body of people with a particular purpose, especially a business, society, association, etc.