If I believe that the reason an Agent and end-user isn't getting email alerts is because they clicked the link, at the bottom of the alert stating to "unsubscribe," how would I get them to be resubscribed?

Let's assume, this isn't an email, spam, junk mail, wifi, internet, smoke signal, issue. If we can assume that it is just that how would I fix it? I checked the admin panel and can't find where to remove someone from an "unsubscribe list."