I'd call Meetup a different case; as someone who actively uses the service, both as Organizer & Member... I need to know if someone knew signed up to my Meetup so I can send the a message and engage them. Or if a new group started up competing with me (or groups I'm a member of). My own RSVP notifications are obvious. Sorry, but the email Meetup sends is literally integral to the function of the service.