This paper addresses the need of interpersonal privacy coordination mechanisms 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 a field study that illustrates how it can describe privacy coordination mechanisms amongst users of an instant messaging application and a desktop awareness system. The model describes efficient and effective factors that communicators consider in their decisions to use mechanisms for coordination. The Privacy Grounding Model aims to help designers reflect on how their system supports, or fails to support, people's need for lightweight and distinctive privacy coordination mechanisms, and in particular how communicators within the system create and use privacy border representations for grounding their needs to interact with each other.