Q. Can I use Remote Desktop Services in a workgroup without a domain?
A. There are two licensing methods for Remote Desktop Services, per-user and per-device. If using per-user then a domain must be available as it is the domain account that is tracked via the per-user licensing. If you only have a workgroup you must use per-device licensing.
Note that in small environments the RDS licensing server and the RD session host can be the same server.