This research study focuses on school leadership groups taking part in an action research project (AR project) within schools in a Norwegian municipality. The study aims to show and discuss how action research (AR) adopted in school change can help build collective leadership capacity in school leadership groups. Combined with the theory of expansive learning, the theories of critical participatory action research and practice architectures frame the study. The study identified two essential actions for building leadership capacity in school leadership groups: "performing an empirical and historical analysis" of the problem space worked on and "conducting collective reflections" regarding their experiences in the development work process. Furthermore, the study shows how external leadership supervisors can contribute as critical friends in ways that may be significant for capacity building in school leadership groups. The study concludes with two implications related to collective capacity building for school leadership groups and one methodological implication for performing action research.