The "Email address" field in the RSVP form is required. However, it is not required that the email address be valid. If any of your guests do not have an email address or prefer not to provide theirs when filling out the form, they can simply enter a "dummy" email address. An example of a suitable dummy address would be firstname.lastname@example.org. The "Email address" field in the RSVP form will recognize this as a properly formatted email address and accept it to complete the form.
Please note, however, that since all names and email addresses entered in the RSVP form are recorded in the contact list section of the "Email Guests" messaging feature, any dummy email address entered will show in the contact list. Therefore, you will not be able to send any messages to such guests using that address; you will need to manually enter their actual email address if you have it. Learn more about the "Email Guests" feature.