This can simply be included in the wording along with your phone number or email address or it can be included as a stamped RSVP postcard that your guests can simply drop in the mail to let you know they're coming.
Conducting an internet search for "post secrets," you'll find a huge number of copy-cat sites that now offer the same approach as PostSecrets, using the same general format for the art on each "postcard."
If you search Safercar.gov and find your Toyota Sienna does have a recall and you have not received a recall postcard, most likely the manufacturer doesn't have your current mailing address.
The PostSecret community members would create their own postcard featuring an image or artwork, and over the image they would print their deepest secret that they had never revealed before.
You can place the postcard in a pocket in the scrapbook so you can see both sides or buy two copies of the postcard so the layout can show both the front and back at the same time.