I used to run a local web marketing meetup in East Idaho. It grew to about 260 people! We used meetup.com and I loved that...

Then I discovered that Facebook Groups and setting up events there worked just as well for us. RSVPs, reminders, notifications, etc all built in.

I since retired that meetup due to moving out of the State for a short time, but if I were to start again I would probably still use Facebook.