That sounds very unlikely to me, but just to be sure: Can a member of 'Domain Users' join a computer to the domain (granted that he has the local administrator account)?
The instructor said it, and it sounds very wrong. I tested it and I got 'Access denied' when I tryied to supply a regular users credentials. Am I missing something here?
Update: You get Access Denied if you already have a computer with that name in AD. If you delete the account, any user with local Administrator account could join the computer, auto creating an account in AD. UNBELIEVABLE.