Using POST method in meta box, no results

Firstly, have you inspected the meta box html from the browser?

You may find that the form element that you have above is missing. This is because you cannot have nested form elements:

<form>
      <form>
      </form>
</form>

All of the meta boxes are already inside of a form element, so you cannot specify another form element inside.

One solution would be to use a Javascript AJAX function to submit your adding of usernames:

Javascript Part:

            jQuery.ajax({
                url: 'http://yoursite.com/wp-admin/admin-ajax.php',
                dataType: 'json',
                //type:'POST',
                data: {
                   'action':'your_ajax',
                   'fn': 'add_users_to_project',
                   'data': data
                   },
                success: function(results){
                    //console.log(results);
                    //display your added usernames
                },// end of success
                error: function(errorThrown){console.log(errorThrown);}
            });// end of ajax

PHP part:

class your_ajax{

//add actions and filters in constructor
function __construct(){

    //admin or front end?
    //add_action('wp_ajax_nopriv_your_ajax', array( $this, 'ajax_function' ) );
    add_action('wp_ajax_your_ajax', array( $this, 'ajax_function' ) );

}

//handle ajax calls from 
function ajax_function(){
   // the first part is a SWTICHBOARD that fires specific functions
   // according to the value of Query Var 'fn'

    //feel free to use this file for any wordpress ajax that needs to be done
    switch($_REQUEST['fn']){

        case 'add_users_to_project':
            //$output = $_REQUEST['data'];
            $output = $this->add_users_to_project( $_REQUEST['data'] );
        break;

        case 'delete_users_from_project':
            //$output = $_REQUEST['data'];
            $output = $this->delete_users_from_project( $_REQUEST['data'] );
        break;

        default:
            $output="No function specified, check your jQuery.ajax() call";
        break;
    }

   // Now, convert $output to JSON and echo it to the browser
   // That way, we can recapture it with jQuery and run our success function
        ob_clean();
        $output=json_encode($output);
        if(  is_array( $output )  ):

            print_r( $output );
        else:

            echo $output;
        endif;
        die();

}


function add_users_to_project( $data ){
    //do whatever
    return $response;
}

function delete_users_from_project( $data ){
    //delete whatever
    return $response;
}


}//end your_ajax class

$your_ajax = new your_ajax();

Another option would be to add a function to the save_post action hook which will fire when the project post is saved. Then simply display all the users in your meta box.

add_action( 'save_post', 'your_save_function' );
function your_save_function(){

//do stuff with global $post and $_POST

} 

Sorry for the long answer…Hope you find a solution suitable to your needs.