Not sure if you have seen this quick YouTube video from Mike Agerbo, of Get Connected fame. He gives a two minute review of Groove 2007 and OneNote 2007. Its a good pointer if you are trying to explain Groove 2007 to someone for the first time. Although it implies that you need Communicator in order to use instant messages, its a good quick review.
link to this page: http://blogs.technet.com/groove/archive/2008/04/03/groove-tube.aspx
link to this page:http://blogs.technet.com/groove/archive/2008/03/26/updates-to-groove-web-services-helpers.aspx