If you have unsubscribed a subscriber from a Mailing List by mistake, and want to retrieve that subscriber, simply go to "View unsubscribed members" at the top right of your Mailing List, and you'll see the complete list of unsubscribed email addresses together with the reason or method by which they were unsubscribed.
If they chose to unsubscribe themselves, you may not re-add them to your Mailing List. They must re-subscribe themselves by going to your sign-up page. If this is not possible, contact us by emailing us at firstname.lastname@example.org to re-add them to your Mailing List.
However if it was you who removed them from the Mailing List, you can re-add them by clicking on the grey arrow button next to the subscriber and clicking "Resubscribe".