Child tenants
Settings > Tenant > Child tenants lists the tenants below this tenant (columns Tenant ID, Database, Actions). The toolbar has New tenant, Attach tenant and Restore tenant plus the usual Export to Excel, Export to PDF and Refresh Data. A row opens the page of the child tenant, whose breadcrumb is Provisioning: there you grant users of this tenant access to the child tenant, see Admin provisioning.

Creating, attaching and restoring a tenant needs the role TenantManagement.