Aristotle (in the Nichomachean Ethics), Machievelli (Prince), and Shakespeare all commented, insightfully, on the nature of groups. It was not, however, until the 20th century that researchers began studying groups scientifically. The following timeline stretches from 1890 to 1960.

1890: James puts forward several theories that explain various social psychological processes that are relevant to groups, including social identity.
1895: LeBon publishes Le Psychologie des Foules
1897: Durkheim discusses the impact of groups on social behavior, particulary primary groups. He publishes his classic work Suicide, which explains how an individualistic action can be explained through reference to social forces.
1897: Triplett publishes the first laboratory study of a social psychological phemononon
1907: Cooley publishes work dealing with social organization, structure
1908: The first two social psychology textbooks are published; both are titled Social Psychology.
1918: Thomas and Znaniecki publish their classic Polish peasant in Europe and America
1920s: Elton Mayo and his colleagues study productivity in the Hawthorne plant; they discover that group processes dramatically influence production.
1924: Durkheim and Allport debate the reality of social products, including groups.
1928: Thurstone publishes a precedent-setting paper entitled "Attitudes can be measured."
1934: Moreno presents sociometry
1936: Sherif demonstrates that a purely social phenomonon--a social norm--could be created in a laboratory.
1937: Lewin, Lippitt, and White study group members' reactions to leaders who adopt autocratic, demoncratic, or laissez faire styles of leadership.
1939: A group of researchers at Yale University publish data suggesting aggression is caused by frustration (Dollard, Doob, Miller, Mowrer, & Sears, 1939).
1943: Whyte uses participant observation to study urban street-corner gangs.
1943: Newcomb examines the impact of social pressure on attitudes among students at Bennington College.
1946: Bales begins work on IPA
1953: A group of researchers at Yale publish the results of a programatic study of attitude change (Hovland, Janis, & Kelley, 1953).
1954: Publication of Allport's timeless analysis of prejudice and stereotyping. Also, the first "modern" edition of the Handbook of Social Psychology is published.
1957: Festinger initiates two decades of research on attitude change with the publication of his book A theory of cognitive dissonance.
1958: Heider presents a theory of "commonsense psychology" that provides the basis for all attribution theory and research.
1959: Thibaut and Kelley publish a general theory of social enchange and interpersonal relations.