Right-click on your VM in the Administrator Console and the new (in Beta 2) vm “tag” feature appears. Enter a tag for the VM here. Now you have a way of grouping vms that have tags in the "Virtual Machine Views" section, or sort the Virtual Machines view by selecting Tag from the “group by” list. Mighty handy as a way of tracking self-service vms, for example.

 

To set a tag on a vm, right-click and view properties. On the General tab of the vm enter a value in the "Tag" field.

 

And, because this is VMM, where everything you can do in the GUI you can do on the command-line, to query on the tags, on the VMM shell command line use:

 

get-vm | where {$_.Tag -match "<tag name>" }