martinmalgor
You either need to allow Users to register themselves or have the Agents manually send registration emails to the Users.
If you allow Users to register themselves they simply go to the Client Portal, click Sign In, click Create an Account, and complete the Registration form. All they need to do is make sure the email address they use to register matches the one they used to create the Ticket(s). Once complete, the User will be able to login to the Client Portal and see all previous Ticket history.
If you want Agents to register Users then they must go to Agent Panel > Users > User Directory, find the User, check the box next to their name, click More, click Register, click OK. Once complete the User will receive an email with a link to click to finish the Registration.
Cheers.