In this paper, we describe a user-centric access control process for devices and services in smart space environments. The M-Zones Access Control (MAC) Process is driven by context information relating to the activities of the users present in a smart space, and by user-defined policies reflecting these users' preferences. As well as dynamically assigning access rights in response to context changes, the process provides for automatic reconfiguration of resources in order to protect a user's privacy as other users enter/leave his/her vicinity. To illustrate the implementation of the process we discuss its realisation in a test bed emulating an office-based smart space.