How to add user to local administrator group of remote computer

First we will take our input.

We need domain in order to search for the user in the Active Directory the remote computer we want to add the user as admin into and the username of the user who want to be the admin.

Now we want to get inside the local Administrator group inside the remote computer.

In order to do it we will use the [ADSI] adapter.

Explanation:

ADSI stands for Active Directory® Services Interface. Despite the name, ADSI isn’t actually specific to Active Directory.

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ADSI can connect to a variety of directory services, including Windows NT®-style directories—not just Windows NT domains, mind you, but also to the local Security Accounts Manager (SAM) on standalone and member computers. 

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As the name implies, a type adapter "adapts" a .NET Framework object type into a more consistent, and sometimes simpler, format.

For more information you can visit the great post of Don Jones at:

http://technet.microsoft.com/en-us/magazine/2007.06.powershell.aspx

We will use this adapter in order to access the local administrator group in the remote computer:

After we have the group we will add the user: