Michele Pinassi

If you want to show your meetup’s groups activities on you wordpress blog, here’s the widget right for you. 
Just get the API key from Meetup.com and choose which groups to show: you’re done !

WP-Meetup-Activity

WP-Meetup-Activity display your groups latest activities (discussions, photos...) and events in a sidebar widget

Rick Radko

A simple widget for use with a <a href="http://meetup.com" rel="nofollow ugc">Meetup.com</a> group. The plugin adds a widget that shows a link to your Meetup group.
Simply enter your Meetup group name and it’s URL and save. You can display the widget with or without a title.
Support
Support for this plugin is limited to fixing confirmed bugs and improving the plugin with enhancements that can be reasonably accommodated.

R3DF Meetup Widget

A simple widget for displaying a link to a meetup.com group.

WP-Meetup-Activity vs R3DF Meetup Widget