Nowadays, systems are more and more open, distributed and collaborative. In this context, access control is an important issue that should be studied, specified and well enforced. This work proposes a new access control model for collaborative systems: "PolyOrBAC". On the one hand, we extend OrBAC (Organization-Based Access Control Model) to specify local as well as collaboration access control rules; on the other hand, we enforce these security policies by applying web services mechanisms (XML, SOAP, UDDI and WSDL). Then, we present a representative scenario of secure collaborative applications. Furthermore, we propose a XACML-based implementation of PolyOrBAC and we discuss the most important approaches that emphasize access control in collaborative environments.