Group therapy is psychotherapy that involves working with more then one person at the same time. Different types of group therapy include support groups, mindfulness skills training groups, or personal growth groups. The exploration of interpersonal relationships is extended to all group members.
Yalom defines the details of therapeutic factors at play in groups that include universality, altruism, instillation of hope, imparting information, corrective recapitulation of the primary family experience, development of socializing techniques, imitative behaviors, cohesiveness, existential factors, catharsis, interpersonal learning, and self understanding in his book titled Group Therapy. You can find my offerings for group therapy under the workshops heading.