Register subpage
add_action('admin_menu', 'register_my_custom_submenu_page2');
function register_my_custom_submenu_page2() {
add_submenu_page( 'edit.php', 'Asignar usuarios', 'Asignar usuarios', 'read_private_pages', 'users', 'my_custom_submenu_page2_callback' );
}
function my_custom_submenu_page2_callback() {
?>
<h1>Asignar usuarios</h1>
<form method="GET" action="#">
<input type="hidden" name="asignar" value="true">
<input type="submit" class="buttom" value="Asignar">
</form>
<?
if(isset($_GET["asignar"])){
die("asignando...");
}
}