A multi-tenant identity management (IDM) system enables IDM functions to be performed relative to various different customers' domains within a shared cloud computing environment and without replicating a separate IDM system for each separate domain. The IDM system can provide IDM functionality to service instances located within various different customers' domains while enforcing isolation between those domains. A cloud-wide identity store can contain identity information for multiple customers' domains, and a cloud-wide policy store can contain security policy information for multiple customers' domains. The multi-tenant IDM system can provide a delegation model in which a domain administrator can be appointed for each domain, and in which each domain administrator can delegate certain roles to other user identities belong to his domain. Service instance-specific administrators can be appointed by a domain administrator to administer to specific service instances within a domain.