This paper deals with the structuration of a territorial interest group, made up of employer representatives, mobilized in favor of the creation of a metropolitan institution in Marseille. This mobilization, which began in the mid-2000s, fits alongside the vote of the MAPTAM law in 2014 creating the Aix-Marseille-Provence (AMP) metropolis. At the crossroads of a sociology of interest groups and a sociology of territorial reforms, this article highlights the logic of the structuration of a small business elite, whose wide resources, both expert and relational, did not initially enable them to guide the choices of local political actors, who were mostly reluctant to support an integrated metropolitan institution. It was not until 2012 and the decisive intervention of the state, then in search of local support to overcome the opposition of elected officials, that the role of these employer representatives was legitimized. If this legitimation was more strategic than ideological, it nevertheless has had very concrete effects, leading to the integration of a certain fringe of economic interests within the AMP authorities.