Was having a similar problem. I had new builds of an old up and the new builds were not updating in TestFlight. The above solution by campbeld worked for me. I went into app details in TestFlight, and clicked "Stop Testing". Then I accepted the invite email, followed the prompts and it re-instualled the new build for me. .

For the initial screen, I added a hidden label, which I only display when the user opens the app for the first time, and the label has instructions telling the user how to proceed. I am also in the process of adding a help button on each page, which will segue to a page with detailed instructions.

Thanks, Claude31!