Add Invite / appointment in user's Calendar.

how can edit the confirmation email to include the ability for the person to add the invite/appointment in their calendar?

I am not sure if I have understood your requirement correctly. Do you mean to say that you want your users to be able to add invite in their Calendar form the auto-responder email which is sent after form is submitted?

Unfortunately it is not possible to add event or invite in user's calendar. Please note that when email includes a date, most of the email client will display an option to add to calendar when user mouse hover on date in email.