When a subscribed contact clicks the unsubscribe link in your Mailchimp campaign or when you manually unsubscribe a contact, that contact is immediately unsubscribed. We know accidents can happen so here are a couple of ways to resubscribe a contact in Mailchimp.
First and foremost, respect the contact’s wishes to be unsubscribed from your audience list.
I have this person right here who unsubscribed through a campaign email. You’ll see it says unsubscribed.

If I go to actions here, select the box next to that person, and click resubscribe, I can do so.

However, you will see an error that says, “contact was not included, that I already unsubscribed and that I did not opt into marketing cleaned.”

Because this person opted out, I can’t just resubscribe them this way. However, as the list owner, if you unsubscribe them for various reasons, such as low engagement. You can manually resubscribe them through this same process, but you won’t see the error.
Let’s say that the unsubscribed contact here communicates that they’d love to be back on our list and want to know how to resubscribe.
You can go to your signup forms, click on Form Builder, and send them this URL, or you can fill out the form on their behalf.

When you do so, remember to obey applicable laws in your area. If you have any questions about this process or anything related to Mailchimp, post them in the comments below.
Do they get an email to say they have subscribed if you complete the form yourself?
They may if you have double opt-in turned or send final welcome email turned on.