While trying to add a new user to grant PowerShell script execution policies, I got this error message:
Add-SPShellAdmin: You need to have Machine administrator priviliges to run this cmdlet
Although you are in the local server Administrator group, You must run the SharePoint Management Shell as Administrator. Simply right-click on SharePoint 2010/2013 Management Shell >> Choose “Run as an administrator”!
Alternatively, you can disable the UAC to avoid these kinds of issues. You may face this issue when you try to run cmdlets such as: Add-SPShellAdmin, Backup-SPFarm, New-SPWebApplication, Set-SPManagedAccount, etc.
Related article: How to disable UAC in Windows 2012?