One thing we always want to stress with Edmodo is everything you post in your groups are private by default.  We did however want to give teachers options to make posts public on a case by case basis to share with the world.  Every group you create has a public page

To get to your groups public page click the management icon next to your group, then click the "public" link in the drop down menu.

Click the public link on a post to make an item public.

Click the red x to remove an item from your public page.

This is what your public page will look like.  It has an RSS feed too.