Conceiving of a group as a dynamic whole should include a definition of group that is based on interdependence of the members (or better, the subparts of the group). Kurt Lewin (1951: 146)
We mean by a group a number of persons who communicate with one another often over a span of time, and who are few enough so that each person is able to communicate with all the others, not at second-hand, through other people, but face-to-face. George Homans (1950: 1)
To put it simply they are units composed of two or more persons who come into contact for a purpose and who consider the contact meaningful. Theodore M. Mills (1967: 2)
A group is a collection of individuals who have relations to one another that make them interdependent to some significant degree. As so defined, the term group refers to a class of social entities having in common the property of interdependence among their constituent members. Dorwin Cartwright and Alvin Zander (1968: 46)
Descriptively speaking, a psychological group is defined as one that is psychologically significant for the members, to which they relate themselves subjectively for social comparison and the acquisition of norms and values, ... that they privately accept membership in, and which influences their attitudes and behaviour. John C Turner (1987: 1-2)
A group exists when two or more people define themselves as members of it and when its existence is recognized by at least one other. Rupert Brown (1988: