While the service that Meetup.com provide is excellent, their Privacy Policy is not. It's hard to believe that an established internet company in this day and age would engage in sending unsolicited emails... but that's exactly what this company does. And they have quite a reputation for it.
Top tip: If you sign up -- Use an email address that you don't check!
It's very disappointing indeed that they have such a large disregard for their customers. You thought Facebook was bad? It's nothing compared to this company!