Allow RDP for user from commandline
Sure, you can use the NET command:
NET LOCALGROUP "Remote Desktop Users" domain\jscott /ADD
This would add the domain user domian\jscott to the local group Remote Desktop Users. If you'd like to add a non-domain user, simply leave off the domain prefix:
NET LOCALGROUP "Remote Desktop Users" keyoke /ADD
This would add the local user keyoke to the local group Remote Desktop Users.