In recent years, parties in many parliamentary democracies have radically re-shaped what it means to be a party member, most importantly by giving them more direct say over party decisions. This article explores some of the implications of these changes, asking whether the costs and benefits of membership have an effect on which supporters take the step of joining their party. In particular, it considers the impact of net membership benefits on membership demographics and on members’ ideology. The article investigates these questions looking at patterns of party membership in 10 parliamentary democracies, using opinion data from the European Social Survey and data on party rules from the Political Party Database project. Our analysis shows that party supporters are more sensitive to political benefits than to financial costs, especially in terms of the ideological incongruence of who joins. As a result, parties offering higher benefits to their members have lower ideological and demographic disparities between members and other party supporters. This is a positive finding for party-based representation, in that it suggests that trends towards more inclusive decision-making processes have the potential to produce parties with memberships which are more substantively and more descriptively representative of their supporters.