Is there an open source posting host for organizing and sharing information and events?

Welcome to Fluther! Have you considered creating a Facebook page for your group? You can do all of these things (invite people to events, share other info, have other people post to the page, etc.) on a Facebook page, it’s free, and social media is a great way to get information out to a community.

Sounds like you could start a subreddit on Reddit .

Facebook is not open source. Reddit is though.

You could take a look at http://libreprojects.net to see if there’s something listed that you could use.

What @Fly wrote. Or on Google+; Google have group-shareable apps like calendar.