I added a Windows Server 2008 R2 x64 to my 2008 domain. When I attempted to add the domain admin to the local administrator group I got an error that it already is there!
But when logging in as the domain user I did not have admin rights. I kept seeing access is denied. This is what I had to do to resolve the issue.
1. Remove the account from the admin group via the command line using this command:
net localgroup administrators domainname\userid /delete
2. Then going to start –> run and type: control userpasswords2
3. Click add.
4. Type in the username and domain.
5. Select Administrator
You are done! Hope this helps!