I've run hundreds of events through refreshmiami and the garbage ticketing system meetup gave us years ago made us rely on eventbrite. We get way better email tools and analytics on eventbrite. From my org alone, meetup has lost out on thousands of $ in fees.
As for building a new one, would be interesting to explore which angle on the market do you tackle? Eventbrite? FB groups? etc.
You make me wonder if Eventbrite might be that solution, do they not have groups?
Still, a better one might exist, and since there's no friendsters on Meetup (tmk) switching costs are lower if people only need to create an account and join a group (i.e. where Meetup pretty much stopped developing).
A few reasons we haven't left eventbrite for our own woocommerce or other based platform for ticketing (the events calendar pro plugin has some for wordpress): check-ins for events. None of the other solutions have as streamlined of a system or as many simple embeddable integrations for the ticket sales.
As for building a new one, would be interesting to explore which angle on the market do you tackle? Eventbrite? FB groups? etc.