Try this, I have worked with command signs for a little while now and found a way to make the sign message the player do the following command
/commandsign setcommandsign false msg @p
and then right click a sign and it will bind it to the sign. The false in the middle is saying if you want the sign to be a one time use or not (for message signs I prompt for multi-use) The @p is telling the sign to include the username of which ever player interacted with the sign and of course is where you would put the message you would like the server to message the user.
Hope this helps you out