2P data management systems provide a scalable alternative to centralized architectures. Their adoption, however, is limited by the lack of possibility to control the access to the resources stored in the system. We address this problem in the case of structured P2P networks, in particular, when the system is used in a collaborative working environment. We analyze the problem assuming a simple threat model and we systematically explore the solution possibilities. We design and compare access control enforcement techniques which realize the desired functionality by constructing independent networks or by implementing access control at query or at response time.