Recent decades have witnessed dramatic changes all over the world. One major trend is the proliferation and diversification of actors, forums and their arrangements to address global governance challenges, which has led to fragmentation in global governance. However, such contested multilateralism has a positive dimension, as the emergence of informal multilateral institutions claiming a major role in defining the global governance agenda creates alternatives for providing common goods. New arrangements acquire their own actorness and place in the system of global governance. In certain policy areas, there is a clear trend for the new summit institutions’ leadership. The most visible recent cases include the Group of 20 (G20), the BRICS group of Brazil, Russia, India, China and South Africa, and the Asia-Pacific Economic Cooperation (APEC) forum, with APEC gaining importance regionally and globally. These new informal groupings work on their own agenda. They also engage with established international organizations to steer global governance processes. Taken together, the transformative trends in international relations, the emergence of new actors, tensions between exclusive and inclusive clubs, and demands for the legitimacy and effectiveness of the international institutions define the relevance of the study, systematization and comparative analysis of the effectiveness of this model of cooperation among international institutions. This article builds an analytical framework by undertaking three tasks. It first reviews the key concepts. Second, it argues for a rational choice institutionalist approach. Third, it puts forward a hypothesis for research: to compensate for their inefficiencies, summit institutions engage with other international organizations in a mode they regard most efficient for attainment of their goals. The modes of those institutions’ engagement with other international organizations as reflected in the leaders’ discourse should thus indicate the role of those institutions in the global governance architecture, which is imputed at their launch and subsequent evolution. The hypothesis further suggests that the “governing in alliance” mode enhances the effectiveness of the summit institutions; however, those institutions’ use is not mutually exclusive. The modes of engagement with international institutions coexist in the engagement of informal summit institutions with other international organizations. The choice is defined by the policy area and type of organizations. The article concludes with a case study of BRICS engagement with international institutions. The results confirm that the choice of engagement model reflects the forum’s role and place in the global governance architecture. To maximize benefits from cooperation, the BRICS engages with relevant international organizations on agenda priorities at different institutional levels. Two types of engagement are typical for the BRICS: catalytic engagement (exerting an influence for changes in international organizations through endorsement or stimulus, or compelling them to reform) and parallel treatment (creation of the institution’s own mechanisms). By establishing new institutions, the BRICS consistently strengthens its cooperation with other international institutions. Its choice of model depends on the policy area, where it is developing cooperation and the perception of the organization’s relevance to BRICS objectives. BRICS engagement with United Nations organizations and the World Trade Organization follows the model of catalytic influence, whereas with the G20, BRICS engagement based on the model of governance in alliance with multilateral institutions remained unrealized.