As someone who spent 2.5 years working at Meetup during the early '10s (no idea what it's like today, mind you)... But:
Organizers were never able to see anything but basic information about users. Definitely not email. Mostly historic RSVPs, a userpic, and whatever information provided when you joined a group or RSVPed (organizers could ask questions in both cases).
Despite what people seem to believe, the _vast_ majority of Meetups are about topics that aren't technical at all. There have always been technical oriented meetups, of course, and that category used to have a lot of user volume for events, but most of the meetups on the site were for hobbies, support groups, hiking, biking, dogs, etc, etc, etc..
Let me also say that _most people_, at the time, resisted changes to the site. Redesigns (and there were a few attempts while I was there) would result in crushing amounts of support tickets, and social media campaigns threatening the business. It was wild! During this time:
1. Meetup did usability studies (and even paid for them)
2. Ran lots of A/B tests, and did so properly (and by this, I mean, we waited until the statistics gave us actual confidence)
3. Were generally very conservative about roll outs or design changes, must to the chagrin of everyone that worked there.
Meetup also had (and likely very much still has) a really interesting problem. Most people want to passively join a group and show up to a few meetups when their schedule allows. But! Organizers who don't organize meetups, or who aren't into it anymore, mean that the group exists but doesn't do anything and sits inactive. There's no worse feeling in the world than finding a group of potential shared interest holders only to realize that the group doesn't actually do anything, and maybe never has.
And here's the problem. When I was there, we were about 100 employees, and we _all_ were there (not for large sums of money, let me just get that out of the way) because we believed in the mission: "A Meetup everywhere about (al)most anything." This company was completely mission driven!
So! What do you do? You want organizers to have "skin in the game" and be passionate about what they're doing so that the rest of the group actually has a good experience. Well, you charge the organizer! When they run out of passion, they stop paying, someone else maybe steps up! It worked so well, soooo many times, and it was a big piece of the "how do we make money?" problem.
Meetup was so unique back in those days. There were discussion boards, sure. But, we only ever felt successful when actual people went to actual meetups. It was the best feeling in the world to see thousands and thousands of people getting out in the world and meeting their neighbors to do simple things (and not so simple things, like finding the 5 other people in your state that suffered from the same chronic medical conditions), and if you were ever in the office on Broadway, you couldn't forget that feeling, cause you saw the thousands of pictures of happy people meeting up all over.