Structured P2P networks are vulnerable to the sybil attack. In this attack, a misbehaving person generates a huge number of node identifiers and possibly chooses some of them in order to disrupt availability or integrity in the P2P network. In order to circumvent this attack, one person should be able to obtain only a limited set of identifiers and should not be able to choose them. Moreover, due to the distributed architecture of P2P networks, this limitation should not be managed by a centralized system. In this paper, we propose such a sybilproof distributed identity management system which is based on invitations, and thus rely on social relationships between users.