Do it by using one of the many third party forms.
Try having a look at
http://www.wufoo.com. You can design your own form with whatever you like and rather than a contact and feedback form can call it an RSVP form. You will receive an e-mail when someone completes it and the data will be held at Wufoo. You can set it so that they receive either an e-mail response from your when they complete the form or you can choose to take them back to a special thank you page that you create on your site.
Also try Google Docs - you need to create a Google account, but under the Docs section, you can create as many forms as you like with different templates. All the data is stored as a spreadsheet so that it is very easy to see in a tabulated form. Can't re-direct people back to your site though, but there is a personal greeting that you can set and you will receive an e-mail when anyone completes your form.
With the contact and feedback forms, you just create your form and then copy and paste the code into an html snippet on your site on whatever page you want it on.
Your other option is to use something like a guestbook, where you can copy and paste the code into an html snippet on your site and people can sign this as an RSVP.
The other option is a commenting system such as
http://www.intensedebate.com. It is free and again, you copy and paste the code into an html snippet on the page of your site that you want it on and people can leave comments and you can reply in real time. You get an e-mail each time someone comments and can set the comments to be moderated first before being posted.
Then of course, you can create your form by placing your own html code into an html snippet and depending on where you are publishing your site - MobileMe or an external server, the form results will be e-mailed to you directly and the form will open in the e-mail client or you can use FormMail (server/host option only).
iWeb may be limiting in some respects, but there are lots of options that you can use to get around this.
I use a mixture on my own site and use both Google Docs and IntenseDebate with the two working well together.
