I can completely understand why custom challenge authors should be a subscriber, but it seems cost-prohibitive to require invitees to also be subscribers. Our use-case is improving/monitoring cardio for our workplace. It just simply is not doable to get our members to all subscribe. Ideally, we would be creating club-based challenges. Right now, we must invite everyone to the club, then create the challenge and invite.
This is a lot of steps and not user-friendly design.