By default, when you join a domain, Domain Admins are added to the local Administrators group and Domain Users...
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are added to the local Users group. If you need a domain account (that is not a Domain Admin) to be a local Administrator, then you can add this to the local SAM using Users and Passwords from the Control Panel -- after joining the computer to the domain, but before restarting (you can just cancel the prompt).
At this stage, you will still be operating in the local Administrator context that you began the domain join from, and so still have rights to modify the local SAM.