PowerShell: Add Domain User to Local Administrators Group on All Servers In An OU

A co-worker needed to add a specified user to the local administrators group to all the servers in a specific Organizational Unit (OU), across 3 different sub-domains. Because of the different domains and service account names the script required that I prompt the user for input. After several hours (I am still very much learning PowerShell), this is what I came up with, and it worked like a charm.  Any suggestions/recommendations are greatly appreciated.

This blog was extremely helpful in piecing together the last parts that I needed.