Group therapy is a ubiquitous treatment modality. And its recent recognition as an APA specialty reflects both its empirical bases of support as a clinical intervention, as well as the decades of work that have gone into establishing competency standards for its practice. The present study seeks to add to this literature by examining how one develops into a group therapist, and how to best train group therapists to meet competencies. Specifically, for those who have achieved "expertness" in the practice of group work, this grounded theory (GT) study explores both how have they done so and how such practitioners, in turn, train and evaluate effectiveness in their group therapy trainees. Through analysis of in-depth interviews with nine peer-nominated expert group therapists, essential components of their development included an emphasis on experiential training, personal therapy, didactic coursework, participating in professional conferences, and simply enjoying group work. The study also identifies common competencies that these experts look for to determine effectiveness among their trainees, as well as how these experts determine their own level of effectiveness (and how these processes differ). Finally, the study proposes an emergent theoretical model as to how one develops a professional identity as a group therapist. Study limitations and implications for practice, training, advocacy, and research are discussed. (PsycInfo Database Record (c) 2022 APA, all rights reserved).