One dimension of the growth in informal global governance has been the rapid rise in the number of informal international organizations (IOs). While some of these organizations, such as the Group of Twenty (G20), are now at the centre of global debates across multiple policy domains we know very little about how they govern. We do not have answers to critical questions, including to what extent do informal IOs rely on indirect forms of governance? On what basis do they select IOs as intermediaries? And why do IOs choose to participate as intermediaries? Drawing on an original database of G20 orchestration between 2008 and 2019, combined with interviews with IO officials and G20 negotiators, this article provides answers to these questions. The data highlight the frequency of G20 orchestration, the importance of capabilities and control in intermediary selection, and IO efforts to solicit G20 orchestration. The findings advance debates on informal IOs to consider how they govern, how intermediaries like the OECD can shape G20 agendas, and why IOs participate in these indirect governance arrangements.