This paper addresses interpersonal privacy coordination in the context of mediated communication, emphasizing the dialectic and dynamic nature of privacy. We contribute the Privacy Grounding Model − built upon the Common Ground theory − that describes how connected individuals create and adapt privacy borders dynamically and in a collaborative process. We present the theoretical foundations of the model. We also show the applicability of the model, where we give evidence from two case studies that illustrate how it can describe privacy coordination among users of an instant messaging application and a desktop awareness system. More generally, we believe designers can use the Privacy Grounding Model to reflect on how their system supports or fails to support people's lightweight privacy coordination mechanisms, and in particular how communicators within the system create and use privacy border representations as common ground. Finally we briefly consider the design of interactive and lightweight privacy grounding mechanisms.