Best Wordpress plugin to engage your users and collect their email. You can use header alerts, popups, or exit pages, and it works with any popular emailing service such as Mailchimp or AWeber
I've been playing around with the "Referral Opt-In" section of WPSubscribers recently trying to figure out the best way to use it. Since there was no training for this area, I've put together a video tutorial for you: