I can't think of any other way they can make money without exploiting their users somehow. Either Meetup turns their userbase into machine learning datasets, people spend some pocket change on it, or Meetup doesn't exist. (This same dilemma exists for most non-data center/non-ecommerce internet companies. People feel that a row in someone's database isn't worth paying for even though it took significant labor to create.)

I mean it's not like going to an event in real life is free in the first place. You have to pay for transportation and so on. You spend your precious time. If it's important to you, why be stingy over 2 bucks.