Social Identity research has found prominence in the realm of collective action but lacks an environmental activism focus. This study gathers individuals’ experiences to explore the influences on decision-making processes to join an environmental activist group – the local Extinction Rebellion (XR) group in Stuttgart, Germany. This case study is used to identify patterns in this process, to test the applicability of existing models and to explore the significance of social identity. Activists and past activists were interviewed. The results make it apparent that the existing models are not sufficient to represent social identity processes in environmental activism. The findings are visualized in a suggested adjusted model of collective action, which suggests norms and morals and (politicized) social identity as a twin core influencing collective action. Social identity retains its significance at the center of the model. Furthermore, it is suggested that collective efficacy beliefs anda sense of agency are interchangeable factors influencing the twin core and collective action directly. Injustice perceptions remain essential. Social connections, group identification, group dynamics, participatory efficacy, self-identity, and image are found to play a role occasionally but require further investigation. The convergence of collective efficacy beliefs and a sense of agency and the valuation of factors present especially relevant future research topics.