Sets an affiliate user's role to a specific role when being added as an affiliate

gistfile1.php

<?php
/*
 * Plugin name: Affiliate role on registration
 */

function pw_affwp_set_role_on_registration( $affiliate_id = 0 ) {
	
	$user_id = affwp_get_affiliate_user_id( $affiliate_id );
	$user = new WP_User( $user_id );
	$user->add_role( 'affiliate' );

}
add_action( 'affwp_insert_affiliate', 'pw_affwp_set_role_on_registration' );