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

Kelly Choyce-Dwan

For use with a <a href="http://meetup.com" rel="nofollow ugc">Meetup.com</a> group.
This plugin creates two widgets: one a list of events from a meetup group (by ID or URL name, for multiple groups use IDs); the other shows details about single event (by ID) with a link to RSVP – using OAuth if keys are specified, otherwise just a link to the event on meetup.com. Does require at least an API key (which it asks for on the settings page).

Meetup Widgets

Adds widgets displaying information from a meetup.com group.

WP-Meetup-Activity vs Meetup Widgets