On the phone that worked best, here’s what I did - checked for boarding group at 7:55, and also looked at linked passes. Then I killed the app, and restarted it but left it at the top screen without checking again. Then I suspended (not killed) the app by going to the home screen... at 7:59:58 I re-opened the app, then went into the ride of the Resistance screen, where “join a boarding group” was enabled. I went in, both my wife and I were already selected, then I hit confirm. At that point I had 11, which was great for us as we fly out in the afternoon.
I used an Apple Watch with a digital second hand to see when exactly to reopen the app.
My wife’s phone she was just going in and out of the Rise screen but never saw join a boarding group activate.
We have t-mobile and were sitting in fantasyland with a good cellular signal.