When you send out responses to your customers - the ticket URL would be available to the users depending on the user permission which can be understood by navigating to Admin -> Channels -> Portals -> Settings. So there are two options - logged-in users or anyone with a public ticket URL. 

Considering the question, the option in your settings is "logged in users" who would need to be verified to have an account in your portal so that they could log in and check. There are two options to do this - make sure you send out an activation URL manually from the customers' tab or automate it in Adnin > Workflows > Email notifications > Requester notifications > User activation. Finally, if you would want a quicker alternative, you could guide them to do a password reset on the portal.