Tja!
<script type="text/javascript">
function initRadios()
{
var elms = document.forms["f"].elements, len = elms.length;
while (len--){
if (/^radio$/i.test(elms[len].type) && /^r$/i.test(elms[len].name)){
elms[len].onclick = function()
{
document.getElementById("foo").style.display = "block";
}
}
}
}
window.onload = function()
{
document.forms["f"].reset();
initRadios();
}
</script>
<form name="f">
<input type="radio" name="r">
<input type="radio" name="r">
<input type="radio" name="r">
<p id="foo" style="display:none">
<input name="email">
<input type="submit">
</p>
</form>