In recent years, social media changed the way individuals participate in social movements. While activists demonstrate on the street to fight for a public goal, members of specific movements can also act collective online. Thus, different aspects might influence the formation of collective identity and therefore drive collective action on social media. This study combines the perspectives of social identity- and identity theory in order to examine how members of an opinion-based group contribute to the collective group/social identity formation and therefore, to collective action. To this end, we applied automated text classification techniques to Instagram communication related to the social movement Fridays for Future. Analysing 1,137 comments showed that individuals mainly express Group Cohesion and Emotional Attachment rather than Solidarity by commenting on Instagram. This study further presents a proposed model of collective group/social identity of collective action. Succeeding research aims at enhancing the classification and testing the model