You can add a virtual machine to a cluster in a number of ways.
When you add a host to a cluster, all virtual machines on that host are added to the cluster.
When a virtual machine is created, the New Virtual Machine wizard prompts you for the location to place the virtual machine. You can select a standalone host or a cluster and you can select any resource pool inside the host or cluster.
You can migrate a virtual machine from a standalone host to a cluster or from a cluster to another cluster using the Migrate Virtual Machine wizard. To start this wizard, right-click the virtual machine name and select Migrate.