A Virtual Community is a composition of heterogeneous and independently designed subsystems, focusing on large-scale resource sharing, innovative applications and in some cases high performance computation. The sharing that we refer to is the direct access to computers, software, and data emerging in fields like science, industry and engineering. Several open issues need to be addressed in order to make possible these dynamic environments, such as how to manage access policies to coordinate resource sharing, how to establish a community, how to ensure that member communities respect community policies and so on. Protocols to define how to establish sharing relationships between participants must also be enforced: each member entering in a community has to agree on what it is permitted to do and also what it is obliged to do, Resource sharing must be carefully controlled, with resource providers and controllers defining clearly and carefully what is shared, who is allowed to share and the conditions under which the sharing occurs. Resource sharing is therefore conditional, each provider has to define and publish the conditions under which it makes its resources available. Such conditions (or policies) must always be consistent with the community policies, regulating the entire organization. Furthermore, once resource sharing is defined, the members are obliged to give access to its services according to the stated rules. Because the need of setting up a collaborative sharing of resources is fundamental to many diverse disciplines and activities, our work develops a broad class of strategies to establish an efficient virtual community. The goal of our work is to define an overall architectural framework for a collaborative environment, characterized by a flexible and dynamic structure, where participants can efficiently share resources. We also introduce a mechanism to enforce obligations and delegation of authority to other participants without the need of a supervised management of the entire system. The principle we apply to address decentralized control is based on the concept of sharing rights and duties among all members of the community. In particular, we introduce the concept of witness, that is, a community member that can act as a trusted third party monitoring the exchange of resources between two parties. Unlike other approaches [1, 2], the witness can dynamically change and is not empowered to control all community components, thus it is not a bottleneck. Decentralization of responsibilities is obtained through the use of administrative credentials. Indeed, in addition to the witness, we also introduce the concept of community guard, that is, a member responsible of accepting or refusing the joining of new members. In the talk, besides presenting an articulated approach to policy specification we present a framework for dealing with all the aspects of a community life. Additionally, our framework is complemented with a decentralized mechanism for enforcing sanctions when a member does not comply with community policies. Related work mainly focuses on how to support decentralized policies for managing disparate resources that are not under a centralized control. However, previous work does not address issues such as member entitlements, delegation of entitlements, violation detection, and sanctioning mechanisms . By contrast our approach deals with all aspects and provides also strategies to setup communities in an effective and decentralized way. To the best of our knowledge our work is the first proposing such a comprehensive solution for decentralized management of virtual communities.