Ubuntu Servers :: Mapping UNIX Groups To Windows Groups?

I am currently trying to set up a Samba domain server. In the Samba-HOWTO-Collection I found an
example file.(Point 3.3.3.1) In the explanations of the example below, the author says I need to map UNIX Groups to NT Groups. He writes a shell-script of how one could do it, but when I copy it and then execute it, I get the error:

Bad option: rid=512
Bad option: rid=513
Bad option: rid=514

The other groups do get mapped, just the Domain Admins, Domain Users and Domain Guests dont. This is the shell from the HOWTO:

#!/bin/bash
#### Shell-Skript f ̈r sp ̈tere Verwendung aufbewahren
net groupmap modify ntgroup="Domain Admins" unixgroup=ntadmins rid=512
net groupmap modify ntgroup="Domain Users" unixgroup=users rid=513
net groupmap modify ntgroup="Domain Guests" unixgroup=nobody rid=514

[Code]...