I would not add it in an email. People would be cautions about it with good reason - there is no easy way to tell where the "action" is pointing to; and a customer would not take kindly to being redirected to a site without their knowledge after they click the submit button.
I would highly recommend linking to a site. Or - even better - generate two links, using their email address and the answer in the querystring. i.e.
<a href = "www.yoursite.com/submit?from={$SenderEmailAddress}&answer=yes">Yes</a>
<a href = "www.yoursite.com/submit?from={$SenderEmailAddress}&answer=no">No</a>
That way you can make sure that people arnt accidentally answering twice or answering when they didnt receive the invite.