Access control over resources shared by social network users is today receiving growing attention due to the widespread use of social networks not only for recreational but also for business purposes. In a social network, access control is mainly regulated by the relationships established by social network users. An important issue is therefore to devise privacy-aware access control mechanisms able to perform a controlled sharing of resources by, at the same time, satisfying privacy requirements of social network users wrt their relationships. In this paper, we propose a solution to this problem, which enforces access control through a collaboration of selected nodes in the network. The use of cryptographic and digital signature techniques ensures that relationship privacy is guaranteed during the collaborative process. In the paper, besides giving the protocols to enforce collaborative access control we discuss their robustness against the main security threats.