A stack fusion architecture enables a cloud provider to provide Software-as-a-Service (SaaS) offerings to multiple organizations. Each organization operates a Infrastructure-as-a-Service (IaaS) platform and is associated with an organization domain. A cluster of software/communication services is deployed to each platform. Each IaaS platform is deployed to a data center and includes compute, storage, and network resources and an IaaS operating system. Users registered to an organization domain have access limited to the cluster uniquely associated with that domain. The architecture includes a globally accessible domain-to-cluster map used to map each cluster to the associated domain. A locally accessible user-to-cluster map is stored in each cluster to map that cluster to each user registered to the domain uniquely associated with that cluster. The architecture enables communication between users provisioned on different clusters and registered to different domains without exposing private information to the cloud provider.