Resumen: The European Union has seen the rise of informal groups of states as an increasingly important governance mechanism within its formal structures. Such groups can make decision-making processes more efficient, but they also suffer from a substantial lack of legitimacy in the eyes of the non-members. In this article, we examine how informal groups overcome this fundamental dilemma between efficiency and legitimacy and sustain themselves at the forefront of important policy areas. To this end, we trace the development of what we argue to be a particularly useful case: the E3 directoire in the nuclear negotiations with Iran. The empirical results point to new insights into how directoires – and informal groups in general – can use different types of legitimation strategies to gain and maintain legitimacy. More specifically, the E3 implemented three successive legitimation strategies – detachment, co-optation and integration – using different types of legitimacy sources, in particular problem-solving, institutional adjustments and fostering institutional and policy congruence.