As someone who's used Calendly on occasion, it works pretty well, the pricing seems reasonable. I'm not sure there's enough place in such a relatively small niche in the market (cross-organisation meeting scheduling, because if it's in the same organisation, outlook and google calendar already allow for intelligent scheduling) for many competitors ( especially Cal.com competing with themselves via the open source version, which is just a SaaS, so little value add other than hosting). However integrating payments directly for classes seems like a very smart idea. Nonetheless a billion users seems extremely optimistic.
