Despite the frequent debate about membership decline, some established political parties in Western Europe are experiencing a significant increase in membership. This article examines how institutionalised parties handle this growth and highlights how the sheer number of new members and their otherness create potential pressure to adapt both organisationally and culturally. Based on a multitude of qualitative interviews with local party officials of the German Green Party, this article showcases how a political party tries to maintain its concept of party membership by adjusting its organisational structures and communicating its party culture while facing challenges, particularly in the cultural sphere.