We present here the execution model of a policy-driven middleware for building secure distributed collaboration systems from their high level specifications. Our specification model supports nested collaboration activities, and uses role-based security policies and event count based coordination specification. From the specifications of a collaboration environment, appropriate policy modules are derived for enforcing security and coordination requirements. A policy-driven distributed middleware provides services to the users to join roles in an activity, perform role specific operations, or create new activities. We describe here the design challenges for the middleware and present the run-time structures and protocols supported by it for creating activities, roles, and objects.