Groove 3.1 is going away. Of course that’s not strictly true: if you have it, you can still use it. But Microsoft will no longer be offering support for the product. If you’re running 3.1, you can look at this knowledge base article that describes the situation and offers suggestions on how you might proceed.
It was a good product with a good run, but things keep moving, and now we’re on to Office Groove 2007, with SharePoint Workspace 2010 on the way. If you’re ready to move on to a new version, be sure to read that KB article to get your migration right.