Subscribers can forward your email campaigns to friends and colleagues any time. Here's how:
The campaign is delivered to the subscriber's friends in the form of a forwarded email message. Since the campaign was merely forwarded, the people receiving the campaign aren't subscribers yet. It's a good idea to include a call to action to subscribe.
On your stats page, you can see how many times the original campaign was forwarded, and by whom. You can't see the email addresses of the people they were forwarded to until they subscribe.