Self-answering this one: isEligibleForGroupSession seems to return false no matter what, but it turns out that activating the group activity works as expected anyway (I had isEligibleForGroupSession being true as a prereq for launching the activity, but after removing this check I'm able to go ahead with calling activate() anyway).

Sorry, obviously a typo here, but this is about the iOS 15 beta!

edit: all good! (I didn't realize the call had to be started beforehand) Has anyone actually been able to successfully run the code in the project shared above? It seems that .isEligibleForGroupSession is always false on my device. I'm on the second beta of Xcode and the second beta of iOS 15.