A dual is another player that you give your password to in order for them to log into your account and have full control over it. Generally you do this if you do not have the time to run an account entirely by yourself. Dualling is not built into the game and so you do not have to add a dual, you just simply give them your password.
However you should make sure you absolutely trust a dual before giving them your password, because they will have full control to do anything to your account. It is also against the rules to give your password to someone that already plays on your server.