[Wordpress] Add more users to Auther box

functions.php

/**
 * Add more users to Auther box 
 * Author: Mattias Ghodsian
 * Donate a cup of coffee: https://www.buymeacoffee.com/mattiasghodsian
 * Donate Eth: 0xBBB96204E45D11C9799c6B12E6eE6F0d4A071Ef5
 */
add_filter('wp_dropdown_users', 'add_users_to_dropdown_users');
function add_users_to_dropdown_users()
{
    global $post; // remove if not needed
    //global $post is available here, hence you can check for the post type here
    $admin_users = get_users('role=administrator');
    $merchant_users = get_users('role=merchant');

    $users = array_merge($admin_users, $merchant_users);

    echo'<select id="post_author_override" name="post_author_override" class="">';

    foreach($users as $user)
    {
        echo '<option value="'.$user->ID.'"';

        if ($post->post_author == $user->ID){ echo 'selected="selected"'; }

        echo'>';
        echo $user->first_name.' '.$user->lastname.'('.$user->user_email.')</option>';     
    }
    echo'</select>';
}