In this paper we define a decentralized management mechanism for the dynamic coalition characteristic of virtual organization. We propose an identity-based threshold signature scheme for joint management. We also propose the policy mode for joint management, which uses role-based access control mechanism for authorization policies definition, and voting mechanism for authorization decision-making. Our solution can satisfy the dynamic coalition requirement of virtual organization, and also guarantee the autonomous characteristic of participant organizations and grid entities. Privacy preservation is also provided for grid entities to interact with authorized entities. Authorization revocation mechanism is taken into consideration in this paper. The experimental performance also shows the scalability of the solution.