How to grant Local Administrator rights

The following steps show you how to grant a domain user or local user with Local Administrator rights.

Logon to the machine/server that you want to grant Local Administrator right to.
Open “Run” from the Start Menu. Or alternatively hit [Windows + R] key
enter “dsa.msc” (if you are in a Domain Controller machine) or “lusrmgr.msc” (for any other machine)
Navigate to your domain (if DC) and then to “Built in”
Double click on the Administrator group.
Go to “Members” tab and click “Add” button
Assign the account that you want to grant access with.
Click Ok to complete