Add domain account as local administrator on remote computer

To add a domain account to the local Administrators group on a machine that is domain joined, but is remote from the business network (with no VPN) at the time, and with the machine signed in as the domain user that needs to have the additional rights:

Gain remote access to the machine (TeamViewer, AnyDesk, QuickAssist etc), you can then run PowerShell (as admin) and enter the following command:

Add-LocalGroupMember -Group "Administrators" -Member "[Domain]\[Username]"

The user will then need to log off/on in order to utilise their new rights.