While not all interest groups participate in coalitions, some groups join multiple coalitions to form portfolios of coalition memberships. We test hypotheses that the composition of coalition portfolios increases the influence of interest groups over public policy when: (1) the number of coalitions in a group’s portfolio gets larger; (2) the average size of the coalitions in a group’s portfolio gets larger; and (3) a group’s portfolio improves its position within the overall network of coalitions. We evaluate these hypotheses using a study of 115 interest groups involved in the debate over the Medicare Modernization Act of 2003. The results support hypothesis three; groups gain influence over the policy process when their coalition portfolios increase the extent to which they are situated between other groups in the coalition network. However, the ability of groups to proactively augment their coalitional betweenness may be muted by feedback in the policy process.