Q: How put the correct URL to sub-menu plugin? I'm trying to build my first plugin and building my plugin menu like this:
function group_panel(){
    include('html/example.html');   
    echo "<h1>{$_POST['data']}";
}

function groups_add_menu(){
    if (function_exists('add_menu_page')){
        add_menu_page('Group','Group',8,basename(__FILE__),'group_panel',plugins_url('/img/icon.png',__FILE__));
        add_submenu_page(basename(__FILE__),'Example','Example',8,'red.php');       
    }
}

And my folders are like this
I've the file red.php inside of /plugins/my-plugin/red.php but if I press in my "Example" menu, WordPress send my to the index but the first option ("Group") works.
How can put the URL for my menu?

A: You can not load external file in this way. This argument is for menu slug. If you load the external file it is useless, there will be nothing just except what you will print in file also there will be no WordPress functions at all (because WordPress is not loaded).
You can define a callback function for this then include your red.php in that function.
Firs define a constant for your plugin path in main file (where plugin header is defined) so you can use it later as well.
define( 'MY_PLUGIN_PATH', plugin_dir_path( __FILE__ ) );

Then add submenu page with slug and callback function 
add_submenu_page(basename(__FILE__),'Example','Example', 'manage_options', 'my-plugin-page-slug', 'include_plugin_file');

NOTE- User levels (from 1 to 10) are deprecated use capabilities instead. See Roles and Capabilities
Then in your callback function use the path constant and include the file
function include_plugin_file() {
    include_once MY_PLUGIN_PATH . 'red.php';
}

