Before we get to "Subscribers", a quick recap about the participants in a conversation:
Anyone who has registered and is participating in a conversation by posting messages to it while logged in will also be notified when new messages are posted to this conversation (unless they turned off notifications in their user preferences). These can either be team members in your account or free respondent users (which are always free and unlimited).
You also have the option to allow people to respond to your messages anonymously, by recording their response but opting not to register themselves as respondents when they do so. Their message will be shown as "Sent anonymously". Typically, there would be no way to notify an anonymous person when responses are posted, since they didn't leave their email address.
That's where the Subscribers feature comes into play:
Subscribers to a conversation
You have the option to add subscribers' email addresses in a conversation's settings. This will not register them as users.
Subscribers to a conversation simply receive an email notification anytime a new message has been posted to this conversation.
When a subscriber receives an email notification, it will include a link at the bottom, which they can click to instantly unsubscribe themselves from further notifications on this conversation.
Adding subscriber(s) to a conversation
You can add subscribers to a conversation in the following ways:
In this conversation's "People" settings (see below)
Add subscribers using the ZipMessage Zapier integration
Pre-emptively add 1 subscriber to a conversation that will be created via an intake page by appending a URL paremeter to this intake page
Via our developer API
Add subscriber(s) in this conversation's "People" settings
Open this conversation
Go to this conversation's settings
Go to the "People" tab
Enter email address(s). If entering multiple emails, separate them with a comma.
Note: A conversation can only have subscribers if it is public (anyone with the link can view).