This paper investigates how the appropriation of chat apps by social actors is redesigning digital activism and political participation today. To this end, we look at the case of #Unidos Contra o Golpe (United Against the Coup), a WhatsApp “private group” which emerged in 2016 in Florianópolis, Brazil, to oppose the controversial impeachment of the then-president Dilma Rousseff. We argue that a new type of political activist is emerging within and alongside with contemporary movements: the WhatsApper, an individual who uses the chat app intensely to serve her political agenda, leveraging its affordances for political participation. We explore WhatsApp as a discursive opportunity structure and investigate the emergence of a repertoire specific to chat apps. We show how recurrent interaction in the app results into an all-purpose, identity-like sense of connectedness binding social actors together. Diffuse leadership and experimental pluralism emerge as the bare organizing principles of these groups. The paper is based on a qualitative analysis of group interactions and conversations, complemented by semi-structured interviews with group members. It shows how WhatsApp is more than a messaging app for “hanging out” with like-minded people and has come to constitute a key platform for digital activism, in particular in the Global South.