You can manually add a guest to your Tracking page without sending them an invitation! To do this, open the "Guest List Options" menu on your Tracking page - located at the top right of your guest list - and choose "Add Without Sending." This feature is only available for invitations with RSVPs, so you won't be able to find it on any other card-type's Tracking page.
You will need to enter a name and email address, but manually adding a guest will not send the invitation to this guest. If you don't have an email address, you can enter a fake email address as a placeholder.
You will see the status of this guest as “Added” and you can manually change their status to Attending / Regrets by clicking their name and email address, which will bring up the "Guest Information" window. You can change their status, and then click the save option to complete this update.
Once an "Added" guest has been updated to "attending" or "regrets" they will be able to receive any follow up messages that you send to that status group at the email address you provided.