How to store array in usermeta in wordpress

Question

Subtle difference in your name declaration, which needs to be name="test[]".

<select name="test[]" id="test" multiple>
      <option value="volvo">Volvo</option>
      <option value="saab">Saab</option>
      <option value="opel">Opel</option>
      <option value="audi">Audi</option>
</select>

    update_usermeta( $new_user, 'test', $_POST['test'] );

Edit re: your comment,

$meta = get_user_meta($user_id, 'test'); 
foreach ($meta as $key => $value) { 
    echo $value; 
}