1. A method comprising: in a system in which a cloud provider is configured to provide Software-as-a-Service (SaaS) offerings to multiple organizations, each organization to operate a distinct Infrastructure-as-a-Service (IaaS) platform and being associated with at least one distinct organization domain associated with the organization: deploying to each IaaS platform a cluster of software services, including communication services, operated by the cloud provider, each cluster being uniquely associated with the organization that operates that IaaS platform; provisioning users that belong to each organization on the cluster associated with that organization, wherein the provisioning includes storing user information for each user only on the associated cluster; and enabling users provisioned on different clusters to communicate with each other as if the users were part of a centralized cloud communication service, without sharing respective user information with the cloud provider, wherein the enabling includes: creating a globally accessible domain-to-cluster map to map each cluster to the organization domain uniquely associated therewith; and storing in each cluster a locally accessible user-to-cluster map to map that cluster to each user provisioned on that cluster.