means to be a party member, making it easier and cheaper to join, and giving members greater direct say over party decisions. This article explores some implications of such changes, asking whether membership costs and benefits influence 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 investigation examines 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 toward more inclusive decision-making processes have the potential to produce parties with memberships that are more substantively and more descriptively representative of their supporters.