In the unattended install section of nlite. I am trying to add an auto-login user with a blank password who is a member of the Administrators group. However when I install the user is just a limited user and never has administrator priviliges. How can I make an admin user?

Did you check Unattended with nLite?
Are you doing it exactly as the video?

It was reported not long ago that user names with less with 4 characters has problems with administrator privileges (although it's working fine for me with 3 characters).

I actually have that same problem. I fill in all the information and yet I still have limited privileges. It takes me to the login screen on startup and there's always my account with limited privileges and another account (Owner) with admin privileges. I would like a solution as well

Press SHIFT key when welcome window appear.
This interrupts autologon.
Then select Owner - > Go to Control panel -> User Accounts -> Change Your user privileges and reboot.
That's it.