The procedure for adding hosts to a cluster is different for hosts managed by the same vCenter Server (managed hosts) than for hosts not managed by that server.
After a host has been added, the virtual machines deployed to the host become part of the cluster and DRS can recommend migration of some virtual machines to other hosts in the cluster.
Note: vSphere DRS is a critical feature of vSphere which is required to maintain the health of the workloads running inside vSphere Cluster. Starting with vSphere 7.0 Update 1, DRS depends on the availability of vCLS VMs. See vSphere Cluster Services for more information.