Subscribers are website users which are assigned to an e-Letter list. A user can be a subscriber to multiple lists however each user can only be subscribed to each list once.
Users can only be subscribed to a list by invitation which sends them an email inviting them to join the required list. In the invitation email they will be requested to click on a link inside the email to confirm their subscription. This process is known as 'Double opt-in'.
If the user does not click on the link in the invitation email, they will no be subscribed.
Website users can either subscribe themselves via your website. This will send them an invitation to join the list which has been integrated with your website.
Users can also be manually invited from the manage subscriptions page in the mail e-Letter screen. Click on the [ manage subscribers ] button, then click [ add subscriber ]. Enter the email address of the new subscriber and click [add subscriber ] to add the new user into the system. Then you can send individual invites to any of the installed subscriber lists.