Mailchimp unsibscribes

I occasionally have users who unsubscribe in Mailchimp and then realise they shouldn’t have.

How can I re-add them and keep them synchronised with their Groups status in the future? I’m assuming subscribing them again in Mailchimp won’t keep the sync and I don’t want to force sync all users. Essentially, I want to re-sync one user and have them go through the same process as when they were first added to get emails.

Thanks