This document discusses how human society is organized through social groups. It defines social groups as collections of people who regularly interact based on shared expectations and identity. There are three main types of social groups: primary groups which are small and intimate like families; secondary groups which are larger and less personal like coworkers; and reference groups which people compare themselves to like college freshmen. The document also discusses how social groups can influence members' behaviors and thinking, both positively through a sense of belonging, and sometimes negatively through phenomena like groupthink which ignores alternative perspectives.