How to add a domain user to the local admin group remotely?
psexec will do it remotely combined with 'net localgroup' command.
from command line:
psexec \\remotename -u username -p pwd net localgroup Administrators "domain\user" /add
you can also then batch that or use some scripting to apply to a group of machines. or you could always use powershell.
Another option - you can open up Local Users and Groups for the remote computer using this command:
lusrmgr.msc /computer=computername