As what you might have read the previous article about enabling Remote desktop on Win7 computer, take a look on the command to add a user as member on the remote desktop group.
Remote desktop group is a local group that sets permission to users trying to remotely access a computer. There is a setting in the remote desktop setting to permit remote desktop under the remote desktop group.
To join as member remotely, use PSEXEC to connect to user PC with elevated command prompt and use the following:
NET localgroup “Remote Desktop Users” usernametoadd /add
If it gives success message, then the command went through and you can use the user account to login remotely to that PC.