The founders of group psychotherapy in the USA were Joseph H. Pratt, Trigant Burrow and Paul Schilder. All three of them were active and working at the East Coast in the first half of the 20th century.
After World War II, group psychotherapy was further developed by Jacob L. Moreno, Samuel Slavson, Hyman Spotnitz, Irvin Yalom, and Lou Ormont. Yalom’s approach to group therapy has been very influential not only in the USA but across the world. An early development in group therapy the T-group or training group (sometimes also referred to as sensitivity-training group, human relations training group or encounter group) is a form of group psychotherapy where participants themselves (typically, between eight and 15 people) learn about themselves (and about small group processes in general) through their interaction with each other.
They use feedback, problem solving, and role play to gain insights into themselves, others, and groups. It was pioneered in the mid-1940s by Kurt Lewin and Carl Rogers and his colleagues as a method of learning about human behavior in what became The National Training Laboratories (now NTL Institute) that was created by the Office of Naval Research and the National Education Association in Bethel, Maine, in 1947. Moreno developed a specific and highly structured form of group therapy known as Psychodrama (although the entry on psychodrama claims it is not a form of group therapy). Another recent development in the theory and method of group psychotherapy based on an integration of systems thinking is Yvonne Agazarian’s “systems-Centered” approach (SCT), which sees groups functioning within the principles of system dynamics. Her method of “functional subgrouping” introduces a method of organizing group communication so it is less likely to react counterproductively to differences. SCT also emphasizes the need to recognize the phases of group development and the defenses related to each phase in order to best make sense and influence group dynamics.