Where do I monitor the guest's response?

Start at 123invitations.com
Click on “Manage” (Manage your Invitations) menu.
Login to your account (if you are not already signed in).
Click on “Your Events” tab. Click on "Manage Guests" link present in the desired event and you would be able to see the guest’s response.
You will also be continuously informed via mail in case you have ticked on the checkbox “Notify me when guests RSVP”.