WP Meetup fetches event information from Meetup.com and creates posts for each event.
This is a fine plug-in and will be even better when the fonts used by the calendar widget are made adjustable.
UPDATED 6/5/2014: This is still a great plugin and getting better all the time (and still rough around the edges). I find that some extra styling is needed to make it look neat. And if you already have the plugin - be sure to write down your group colors before upgrading! When I upgraded to ver 2.2.5 from ver 2.1.? (only 3 months later), my colors were lost when I upgraded. Thankfully I opened my calendar page before upgrading so was able to scrape my previous colors off that page. You can see the plugin in action on my site here: http://www.amandagiles.com/wordpress-meetups/
Previous I had reported some minor issues with the plugin, the most noticeable of which was that it uses the URL of the Meetup.com group was displaying instead of displaying the actual group name. That has been fixed.