Hello @MichaelN96 ,
Thank you for reaching out. @Clique_Marketing is right. In order to re-subscribe someone onto to your mailing list, they do have to sign up again. This can be done by sending them the URL to your sign up form. You can find this link by going to your "Sign-up Tools" page which is accessed from the "Contacts" page. Once you are on the "Sign-Up" tools page, click on the "Actions" tab right next to the sign-up form and select "Form URL." Now, a link should pop-up on your screen. From here, you can copy that link and send it in a personal email to your contact. Once the message is received, all the recipient will need to do is click on the link and fill out the form; thus, getting them back on the list.
Here is a screenshot to help guide you: