Did not find Aaron Hartzler and/or Fumiko Suzuki as member of the 'Edge Communications Customer Support' chatter group.

Create a new sandbox, go back and create the users from the new sandbox, recreate the group and do all the tasks. It's repetitive and no one likes to repeat work, but its very quick the second time and if nothing else, reinforces the steps. The fix worked for me, and took way less time than I had already spent fiddling with adjustments, looking for spelling errors, and reclicking the Verify button. 

Not sure if this should have been a sign, but it took about 4 attempts to create the initial playground where I ran into this error, so even though I was able to pass the first lesson, I may have been doomed from the start.