Archived from groups: microsoft.public.win2000.active_directory (More info?)
Sorry I pasted the third line by mistake.
If I use:
dsmod group "CN=HighSec,CN=Builtin,DC=Spozinch,DC=Com" -addmbr
"CN=ROSS JONES,CN=Users,DC=Spozinch,DC=Com"
it succeeds. This of course has the UPN full name, Ross...