I have SCVMM 2008 r2 setup on a Windows Server 2012 r2 Hyper-V server. I am trying to add the physical server it is housed on as a Host, and am getting an error message that the domain computers can't be enumerated, and I am at a loss as to where to go from here to fix it. 

Hi Sir,

You may need to use SCVMM 2012R2 to manage 2012R2 hyper-v host .

This is a similar thread which has been discussed :

https://social.technet.microsoft.com/Forums/systemcenter/en-US/5d4103b5-6491-40d2-b52e-7fffde9580ff/vmm-2012-sp1-supported-to-manage-windows-server-2012-r2-hyperv-cluster

Best Regards,

Elton Ji

Yes, SCVMM 2008 R2 cannot manage / doesn't know how to manage 2012 R2.