Affiliates vs. Customers
In previous versions of the shopping cart software, Affiliates and Customers were two different types of users, with separate logins. As of MMv4, affiliates are simply customers who have been authorized to work as affiliates. They no longer have two separate accounts with two separate usernames and passwords to remember.

 
What happens if an existing customer registers as an affiliate?
Since affiliates and customers are now part of the same account, it will be possible for an existing customer to register to become an affiliate. If this happens, the affiliate options are simply enabled in that customer's account. No other changes are made to their customer account; they have the same SID (System ID) number, can still checkout, log in to their customer account area, etc.

The difference is that they'll now also be able to access the Affiliate Account area in your storefront too. They'll be able to create tracking links and earn commissions for the sales they make.

 
What happens if someone who's not yet a customer registers as an affiliate?
In this scenario, a customer account that already has the affiliate options enabled will be created for the user. The user could place orders in the storefront and log in to both the Customer Account and Affiliate Account areas in the store.

 
Designing the Affiliate Registration Page
Like all pages of the storefront, the design of the Affiliate Registration page is controlled by an HTML template. This template is named affiliate_registration.tpl. It can be edited just like any other webpage, either within the store's built-in template editor, or in a third-party HTML editor, such as Dreamweaver or Frontpage.