Click the "Add" button to create a code for the group membership. In the web app when a user enters this code in the input on the subscriptions page, they will be given the option to subscribe to the group membership.

This is the number of subscribed users who joined the group membership using the code.

You can limit a code by email, so that only users with an email on the list can subscribe using the code. If you don't add any emails to the list, then any user will be able to use the code.