By default, the Affiliates Add On for Paid Memberships Pro generates a new affiliate code for each new user. This code recipe will generate an affiliate code for all existing users on your website.
You will need the Affiliates Add On installed for this recipe to work as expected.

The Code Recipe
Adding the Recipe to Your Website
You can add this recipe to your site by creating a custom plugin or using the Code Snippets plugin available for free in the WordPress repository. Read this companion article for step-by-step directions on either method.
To run this script, add /wp-admin/?mypmpro_create_affiliates=true
to the end of your URL.
Once the page has finished loading, the respective affiliate codes will have been generated.