Add a role to all users

add-role-to-users.php

<?php
//Uncomment the following line for converting one role to another
//add_action( 'the_post', 'convert_subscribers_to_executives' );

function convert_subscribers_to_executives() {
    //Note: this only works by navigating to the page with ID 5883 on the site
    //to kick off the loop that updates all of the users.
    if (get_the_ID() === 5883) {
        //get all subscribers from this WP site
        $blogusers = get_users( 'blog_id=1&orderby=nicename&role=subscriber' );

        //Foreach user: add the Executive role (slug: executive) **in addition to** the current role.
        foreach ($blogusers as $user) {
            //wp_die(json_encode($user->ID));
            $addMemberToGroup = new WP_User($user->ID);
            $addMemberToGroup->add_role( 'executive' );
        }
        return;
    }
    else {
        return;
    }
}