What constitutes the "completeness" of an app for eligibility in testflight?

I am trying to get my app into testflight for testing with "external" beta testers. The app is nowhere near complete, it is totally non-functional, and is intended as a starting point for my team to discuss UI elements. Is my app ineligible for test flight until it constitutes a certain level of "completeness" by Apple's standards? Does anyone know what those standards are? Do they differ from the requirements of completeness for an app on the store?