wpmem_update_user_role()

// Basic usage of how the function is applied:
wpmem_update_user_role( $user_id, $role, $action );


/*
 * This practical example below will set a role for 
 * a user when they register. In this example, it 
 * will set the role to 'my_custom_role'.
 *
 * The example uses the 'wpmem_post_register_data'
 * action to apply the membership at registration.
 * @see: https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_post_register_data/ 
 */
add_action( 'wpmem_post_register_data', 'my_set_user_role' );
function my_set_user_role( $fields ) {

    $user_id = $fields['ID'];
    $role    = 'my_custom_role';
    
    /*
     * Set product access. Note how the optional $action
     * is not used because we are just setting a single
     * role. You could "add" another role to the user's
     * existing role (which in this example would already
     * be the WP default role from WP's general settings).
     */
    wpmem_update_user_role( $user_id, $role );
}