Well, not to step on toes or anything, but I have extensive PowerShell experience especially in conjunction with multistore and in secure multi-tenant environments and this description just doesn't jive with my experience.
I personally haven't had any issues passing the group parameter to the New-NaUser commandlet at any time. But I did do a quick experiment tonight, just to verify that my suspicions were true, and I was certainly able to confirm my thoughts. Those thoughts being that the New-NaUser commandlet behaves when using vFiler tunneling exactly as any call made to the Ontapi useradmin-user-add API call behaves when using tunneling. To demonstrate that this is the case, I made a quick screen capture demonstrating such behavior:
https://www.dropbox.com/s/kmlud688ygqkjza/NewUserCreationInAVfiler.mov
In the video, I show a vFiler running in an 8.1 simulator. This vFiler has only the root user created inside it at the time. On vFiler0, I show several users created, including one assigned to the root group and a root role granting all permissions. I go on to show that I capture two connections to the vFiler, one utilizing the root account from vFiler0 and another using the created user account that has root privileges. I then demonstrate how I can create a new "TestUser" in the built-in Users group of the vFiler without issue while utilizing the "true" root connection, but the same commandlet fails when used with the non-"true root" account with the exact same API return string as seen by Dieter and myself in other areas.
So...long story short... Until proven otherwise, I'm of the opinion that Dieter's issue is not a failure of the commandlet, a failure of WFA, or any type of user issue, but more so is a fundamental issue with the underlying Ontapi interaction between the useradmin-user-add API call and vFilers. It's certainly not documented that this is the way that the call should function, so perhaps it is a documentation failure instead of a true bug. My money is on that this is not intended behavior however, as I can execute almost any other API call that supports vFiler tunneling using a similarly permissioned non-root account without issue.
To further expound on this issue, I also tested against vFiler0 using the same two accounts (the "real" root account, and an account with all permissions). In this test, you'll see that when ran against the vFiler0 context or root controller, there is no issue creating a new user when using either set of credentials...
https://www.dropbox.com/s/6wv5v07lk1lxdhp/NewUserCreationInVfiler0.mov