When the last VM in a VMgroup is deleted from the vCenter inventory, it's no longer possible to add a new VM to the group with set-DrsClusterGroup. Adding VMs is only possible if minimal 1 VM is already part of the group. Although it is possible via the vCenter/UI. It would be very helpfull if this would be possible from set-DrsClusterGroup.
Found that the below code makes it possible to add the first VM to an empty VM-group. Might be useful to add this method in de set-DrsClusterGroup code for first VM's!
$spec = New-Object VMware.Vim.ClusterConfigSpecEx
$spec.GroupSpec = New-Object VMware.Vim.ClusterGroupSpec[] (1)
$spec.GroupSpec[0] = New-Object VMware.Vim.ClusterGroupSpec
$spec.GroupSpec[0].Operation = 'edit'
$spec.GroupSpec[0].Info = New-Object VMware.Vim.ClusterVmGroup
$spec.GroupSpec[0].Info.Vm = New-Object VMware.Vim.ManagedObjectReference[] (1)
$spec.GroupSpec[0].Info.Vm[0] = New-Object VMware.Vim.ManagedObjectReference
$spec.GroupSpec[0].Info.Vm[0].Type = 'VirtualMachine'
$spec.GroupSpec[0].Info.Vm[0].Value = 'vm-25'
$spec.GroupSpec[0].Info.Name = 'ESX67VMs'
$spec.DrsConfig = New-Object VMware.Vim.ClusterDrsConfigInfo
$spec.DpmConfig = New-Object VMware.Vim.ClusterDpmConfigInfo
$modify = $true
$_this = Get-View -Id 'ClusterComputeResource-domain-c7'
$_this.ReconfigureComputeResource_Task($spec, $modify)