<?php var_dump($_POST) ?>
<form method="post">
<select name="group[fieldname][]" multiple="multiple" size="5">
<option value="value1">Option 1</option>
<option value="value2">Option 2</option>
<option value="value3">Option 3</option>
<option value="value4">Option 4</option>
<option value="value5">Option 5</option>
</select><br />
<input type="submit" />
</form>