Hello @MichaelL837 ,
It looks like you already spoke with chat and our phone support regarding this, but for the sake of visibility I'll also include the info here.
If you have a contact that's unsubscribed, but wants to resbuscribe, the easiest method will be to have them sign back up using a sign up form, such as a landing page sign up form. The landing page sign up form is the easiest method of doing this since it doesn't require coding knowledge, adding to a website, etc. - it just requires the contact to access the LP's url.
As long as the contact is able to submit their email address (what denotes a unique contact in our system), then the rest of their old information (besides list membership) should be pulled back up at time of resubscription if they don't enter anything new into the sign up form itself.