Just like DRS host groups, you will also need to create DRS VM groups so that they can be used to create VM-Host affinity rules.
The following procedure will help you create host/VM DRS groups:
From the vCenter's inventory home, navigate to the Hosts and Clusters view.
Select a cluster, navigate to Manage | Settings | DRS Groups, and click on Add to bring up the Create DRS Group window.
In the Create DRS Group window, supply a name for the group, choose the type as VM DRS Group, and click on the Add button to bring up the Add DRS Group Member window.
In the Add DRS Group Member window, select the VMs that should be added to the group and click on OK.
The Create DRS Group window should now list the VMs that have been added to the members of the group. Click on OK to create the group.