What are some alternatives?
When comparing Collaborizm and Meetup, you can also consider the following products
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Eventbrite - Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Facebook Local - Discover places to go and events happening near you
Ticket Tailor - Ticket Tailor is an easy to use event ticketing platform that puts you in control.
Agorakit - Agorakit is web-based, open source groupware for citizens.