Windows interoperability - adding a member to an SMB group fails

I've been assessing the progress of Solaris 11.3 WRT Windows interoperability.

I'm starting simple, with a WORKGROUP scenario, basically following what's documented.

When following what's found on https://docs.oracle.com/cd/E53394_01/html/E54797/addmembertosmbgrouptask.html#scrolltoc (for a workgroup scenario) I get the following error which I'd like to understand and sort out:

root@box-01:~# smbadm add-member -m prime builtin\\users

failed to add prime (invalid name)

I believe I have a reasonably minimal working configuration but obviously I'm missing something:

root@box-01:~# smbadm lookup-user users

users: S-1-5-32-545

root@box-01:~# smbadm lookup-user S-1-5-32-545