Microsoft clarifies about an issue that involved the "new version of OneNote 2010 in which the Live Sharing Session feature has been removed from the product". Microsoft says "What we did in OneNote 2010 was to make share notebooks "go to light speed" and start syncing quickly when other people are also editing same page. Means when you're in a shared notebook we're always syncing your edits and once we see someone else editing we start syncing in real-time. We'll kick up our sync interval to sync very often and make sure current page is getting synced in nearly real-time. It's pretty cool to see this in action where people can be writing notes together and we just handle everything. Once the other author stops editing we turn our sync interval back to normal time period and keep a lookout for other people's edits and then we can move into real-time again if we need to. Shared notebooks also have advantage of being in same place where your team shares content, you don't need to send some weird IP address or config file to people, they already have the notebook open."
To get this working you need to put your notebook up on SharePoint 2010 and have your notebook be in 2010 format and just start editing. Or have your notebook on Windows Live SkyDrive.