This paper combines a group biography of the 113 members of the Executive Committee of the French Section of the Communist International (SFIC) with a quantitative analysis of their meetings between 1920 and 1925. It examines the social rationales that shaped the nascent group from the inside. During this period of instability, the communist executive committee was set up through a dual process whereby various social and militant groups were incorporated in the party taking shape, where they either found their place or did not. Initially, discourse was monopolised by leaders with an intellectual profile who dominated the group early on, during a time when a high proportion of its members were women. Our research highlights the gradually rising influence of militants who had previously not been very involved in the French Section of the Workers’ International (SFIO) and its various currents. These militants’ legitimacy was based on their youth, their experiences from the First World War, and in some cases, their working-class backgrounds.