I agree with Nick on this one! I'm an organiser for the Sydney and Brisbane WP Meetups. Meetup is a great platform for organising everything! Not only does it look after RSVP's, ideas from users, messageboards you've got their massive database of users to leverage when you signup. I think if we'd used WordPress for our Meetup group it would've taken us considerably longer to gain attention and to form a group with great numbers.