To Add a windows domain user to a computers local administrator group remotely
first download and install PsExec from
https://technet.microsoft.com/en-us/sysinternals/bb897553.aspx
open command prompt
use psexec to connect to computer
psexec \\computer_name -u \domain\administrator -p password cmd
a command prompt of the remote computer will open on your computer
then type in the command prompt
net localgroup administrators domain\user /add
should return command completed succesfully
Done.
