users sub-collection. As per the usual idiom, POSTing to this collection attaches a user to the virtual machine, whereas DELETEing from it has the effect of detaching that user.
POST /rhevm-api/vms/5114bb3e-a4e6-44b2-b783-b3eea7d84720/users HTTP/1.1
Content-type: application/xml
Accept: application/xml
<user>
<user_name>rhevmadmin@domain.example.com</user_name>
</user>