Scalable and fine-grained Grid authorization requires the move away from gridmap-file based access control and 1-to-1 mappings to individual operating system user accounts. This is recognized and addressed by virtual organization authorization services and user management systems e. g. Virtual Organization Membership Service (VOMS), Local Centre Authorization System (LCAS), Local Credential MAPping Service (LCMAPS) and Community Authorization Service (CAS). They do, however, not address user operating system account management and isolation/sandboxing requirements, such as flexible pooling of accounts while maintaining auditing records. In this paper we compare existing systems which solve the above shortcomings and are currently used in real production grids.