The worst method of doing this is by using an IFRAME, but don't use it, iframes are old. A better way it to include it into your webpage using PHP. For more info: http://us3.php.net/include/
If I understand correctly, you want to include the form page into the page with the join link. This can be achieved be the use of an <iframe>, but I do not think it is necessary to have 2 different pages for this purpose. If you are going to be processing some server-side script, you can always use PHP or ASP includes on your page.
If this isn't the case, please explain your post further.
