Virtual Organizations (VOs) represent a new collaboration paradigm in which the participating entities pool resources, services, and information to achieve a common goal. VOs are often created on demand and dynamically evolve over time. An organization identifies a business opportunity and creates a VO to meet it. In this paper we develop a system for monitoring the sharing of resources in VO. Sharing rules are defined by a particular, common type of contract in which virtual organization members agree to make available some amount of specified resource over a given time period. The main component of the system is a monitoring tool for policy enforcement, called Security Controller (SC). VO members' interactions are monitored in a decentralized manner in that each member has one associated SC which intercepts all the exchanged messages. We show that having SCs in VOs prevents from serious security breaches and guarantees VOs correct functioning without degrading the execution time of members' interactions. We base our discussion on application scenarios and illustrate the SC prototype, along with some performance evaluation.