If you are using the 'open RSVP' setting guests will not automatically be asked to make menu choices when they RSVP. Using this setting means that guests won't be asked for dietary requirements at the RSVP stage as the system isn't aware of which events each guest is invited to.
Once guests have been added to the guestlist with their email address they will be able to log back in and make menu choices. We would recommend perhaps adding a note to the RSVP section of your website advising guests to return after a couple of days of their RSVP to login to the guest area (you can add the button to the footer of your website) to make menu choices. This should hopefully give you a chance to add them to your guestlist.
Alternatively, if you have purchased our email stationery package the guest messenger can be used at anytime to contact guests and enable them to login to the guest area via a link in the email after they have been added to the guestlist.