[Samba] Joining a Samba 3 domain

[Mike]

Hello

I have set up a Samba 3 PDC on RHEL4 using the tdbsam backed. I can log 
in as a user who has been added to the tdbsam database from a 
workstation. When I try to add the workstation to the domain (after 
breaking user connections to the server) I get "The following error 
occurred while trying to joining the domain xyz.com. Access denied.

I havent seen any instructions in the how-tos on how to create the 
Administrative account on the server to accept workstations to join the 
domain?

How do I create security groups and join them to users ie Joe is a 
member of managers, accounts, engineers etc?

Many thanks

Mike