Excerpt from an overview of OneNote shared notebook-based collaboration; see the link below for more details (via the OneNote Facebook page)
Ways to share notebooks in OneNote
In Microsoft OneNote 2010, you can share a notebook so that you can access it on other computers or on the Web, or so that you can work in it together with other people.
Taking notes as a group is a collaborative process. As a collaborative tool, OneNote offers far more than the ability to send static notebook pages via e-mail. Depending on the nature of your projects, you can use OneNote to brainstorm together with other people in meetings, use the notebook pages as a virtual whiteboard, and set up shared notebooks in which everyone can view, add, and edit information.
Unlike other programs that "lock" files for editing by one person at a time, OneNote 2010 lets multiple authors access a shared notebook at the same time. Anytime someone edits to the pages and sections in the shared notebook, OneNote automatically synchronizes the changes so that the notebook is always up-to-date for everyone.
OneNote also maintains a separate offline copy of the notes on each user's computer. That way, shared note-taking participants can continue to edit the notes locally even when they are temporarily disconnected from the network. The next time they connect to the shared notebook, OneNote automatically merges their changes with the changes made by everyone else.