Affiliates can either be imported or added manually on your "Affiliate Signup Page."
Chose the "Sales" option, then "Affiliates."
Under "Users" you will see your existing affiliates you may have, as well as the ability to import affiliates, or sign them up.
To import affiliates, chose the "Import Affiliates" option. You can import up to 500 users to your site at one time.
To sign up new affliliates, chose the "Affiliate Signup Page" option.
You'll then be brought to the Affiliate Details page. Enter your New Affiliate's name, email, PayPal email, and password (they can change this later if they wish).
Then click Save, and you're all done! You now have a new Affiliate! :)