What's the difference between a group and a challenge?
Groups are just ways to organize challenges. They can be used in a variety of ways. For example, a specific "Yearly Running Challenge" could be a group and inside that group there'd be challenges for "2017 run", 2018 run", "2019 run", "2020 run", etc.
A club can also create a group and host a variety of challenges in that single group.
Groups can also be used to host many concurrent challenges and track progress across them all. Sort of like a ragnar race. The group can take on the overall challenge name and then each challenge in that would have have the same parameters, but broken out by team. Challenge Hound has a results page to track progress across groups and we can even add a map at the group level.