While there is a lack of options for customizing or adding additional fields to the website pages - especially in the case of the "RSVP" page - you may consider using your "Blog" page as a "forum" or other space for you to request information from your guests.
To begin creating your "Blog" page as a forum:
1. Hover over the couple's name (ex: Jack & Jill) and select "my website"
2. Then, select the Blog page link. Here, you can enter a name for your blog ("Travel Plans", for example) and a description in the provided fields.
You can also choose to enable the post to allow guests to leave comments or suggestions as well as make them viewable on the blog page when guests view your site.
3. Then, you will need to select Add post under the Manage your blog posts heading.
4. In this section is where you can post the request for your guests as mentioned above.
5. Make sure you select Save once you have finished creating the post.