We have not tested or qualified Parallel Server in a Workgroup, only in a Domain.
If you want to use a Workgroup, it's probably theoretically possible, but you'd essentially need to do what the Domain does for you:
- Synchronize accounts/passwords on all workgroup hosts
- Give administrative privelages to accounts on workgroup hosts (e.g. on the remote host, you need to give Administrator permissions for the remote computer)
This can get pretty unwieldly pretty quickly, and can be very error prone to keep up with. Microsoft recommends moving to a domain when you have more than a handful of machines so that these things become automatic.