So I've been trying to figure out how to make my organic group homepage look nicer than the default "River of News". Well I think we figured it out last night. You need to have the newest version of Organic Groups module, and the Panels 2 module.
Make sure that you enable all the necessary parts, like og_panels, and panels pages, etc.
When you are in the group that you want to modify the homepage of, you click on the 'pages' tab. And there ya go.