While you upload your event content to your organizer dashboard you will be able to view this content in your app immediately. For those listed as administrators for your event (located in the Admin Settings on your dashboard) will have the privilege to view the app on their phones while the event is still in draft mode.
You will not need to publish the app to see what it will look like for your attendees. As you build out your dashboard it will update immediately to your app. And as admin you can view this on your phone.
Your app will look the same as an attendee's app with the exception of the 'admin' section.