How to make a user a local admin on just one computer

1. log in to each "test" PC as the local admin
2. Go to "Control Panel", "User Accounts"
3. Go to "Manage User Accounts", then the "Advanced" Tab
4. Click on the "Advanced" button in the middle of the form
5. Open the "Groups" folder, then double-click on the "Administrators" group.
6. Click "Add", then add the relevant user (Username@Domain)
7. OK (etc) to close all the windows.
8. Log off the local admin account, then log in to the domain user and test.

https://www.sevenforums.com/network-sharing/47604-assigning-domain-user-local-admin-select-pcs.html