To fulfill its promises, the Internet of Things needs to enable collaboration between multiple tenants, providing control of connectivity between devices that belong to different users across different network domains. In this paradigm, security issues that already plague the development of IoT become even more important, and users may have strict requirements on how and when their resources could be shared and in consequence connected with others'. In this paper, we propose an approach to enforce user constraints in dynamic, multi-tenant, software-defined networks of devices. Building on Policy-Based Network Management adapted to user needs, we propose a domain specific language to express policies on shared individual resources as well as shared collections of resources from multiple tenants. We also describe the associated distributed mechanisms to enforce such rules across domains, before showing through evaluation that the solution enforces properly the expressed constraints with minimal time footprint.