The current binary understanding of membership in international organizations (IOs), especially regional organizations (ROs), creates blind spots and biases in our understanding of who matters in IOs, as well as why and how they matter. Existing scholarship primarily looks at full member-states or non-state actors to capture who influences such organizations. Associated states are often portrayed as passive receivers of IO rules instead of active contributors. We address this blind spot and resulting analytical bias by exploring what types of association relationships exist and how they impact IOs. We propose a novel conceptualization of membership that we call memberness. On the level of IOs, memberness is based on the relative openness of organizational boundaries and stratified access via material and ideational contributions. On the level of states, memberness captures associated states’ individual choices to contribute materially and/or ideationally to an IO. Memberness moves away from a purely rights-based understanding of membership (or who you are in an IO) to include a capacity-based understanding (or what you do in an IO). This shift in focus uncovers new channels of influence on IOs. Associated states’ material and ideational contributions to IOs constitute three memberness types: payroller, sponsor, and advisor. We argue that these memberness types impact IOs’ vitality, design, and performance in previously unrecognized ways. We illustrate these types with empirical examples from ROs across the globe and discuss the implications of memberness for IO research programs.