Lists help to organise your Users into groups. To create a list, go to the "Lists" tab and click the "Create a New List" button in the sidebar. You will want to create two lists, one for our API and one for your Newsletter.
The newsletter list won't need any extra configuration other than a name. The API list, however, will need an Identifier. This is what we need, to import the sign-ups from your website onto the system.
Your API list should look something like this: