A noun, though it's often interpreted as a verb. Group therapy is not a performance; it is life, enhanced. It is a method of treatment that involves people interacting as people, with limited supervision. It is more surreal than real.
Group therapy is this:
Exchange of ideas, thoughts, beliefs, etc., without the burdens of expectations and labels. It is more about the exchange of lives than about the exchange of external appearances.
When experienced it does not cheapen life. It enhances it.
See Irvin Yalom.
Reference: The Theory and practice of Group therapy.