If your application isn't approved, you'll receive a letter telling you why so it is possible to reapply. In reality, my passport is currently in inquiry. Either way, it can be a bit stressful, so allow plenty of time to file all the paperwork before you're supposed to depart on your trip.
If you're a citizen of these countries, a unique procedure has to be observed so as to secure a pre-approved invitation. The procedure is similar with just a few differences. You simply go through the typical airport process with no surprises.