This article advances thinking around the semiotic repertoire by exploring how resources are negotiated in the course of interactions mediated by mobile messaging apps, and how this process of co-constructing repertoires is shaped by the affordances and constraints of the virtual spaces in which these interactions take place. Drawing on micro-analysis of ethnographic data from a large funded project, we focus on the closed online networks of two women of Polish origin living and working in the UK, looking specifically at their uses of mobile messaging apps including WhatsApp and Viber. We show how resources come into being through talk and how emergent semiotic repertoires take shape in processes of "repertoire assemblage." In particular, we explore the ways in which technological affordances and pre-programmed signs are harnessed and exploited by mobile interlocutors for making meaning in the course of an immediate interaction, often with longer term impact within a network's emergent repertoire. The study highlights the importance of a dynamic conceptualisation of the semiotic repertoire.