Adding a Azure AD user to local Administrators group

To add an additional administrator to an Azure joined computer.

1, Ensure the new AzureAD user has logged in then out.
2, Login as an admin (AzureAD or local account).

3, Run the following in CMD.

    net localgroup administrators AzureAD\UserAlias /add

4, The next time the user logs in they will have local admin rights.

Use the user alias, not the whole email address, remember that the user must have logged in previously to create a profile on the computer before you do anything.