Groups in Office 365 has rolled out a new feature, Notebook. This new feature allows you to share a OneNote Notebook with you group. It is easy to access the Notebook for your group, just go to a Group from OWA and when you click on the ellipses (the 3 little dots) you get a menu:

Selecting Notebook will open a new tab in your browser and launch OneNote Online, it’s that easy.