How to resubscribe unsubscribers

We currently have three individuals who previously unsubscribed but would now like to start receiving our email campaigns again. However, Constant Contact doesn’t provide an option to send a resubscribe email directly from the platform.

Since we don’t have a subscribe button on our website—and don’t plan to add one—what’s the best way to get these contacts re-added to our subscribed list? Thank you!