Mousecloud is a basic and user-friendly free online collaboration tool which allows you to import documents and collaborate with others online without registration - just share the link of the cloud (or room in online collaboration jargon). The imported documents can be annotated and highlighted. I'm not sure if there is a way to save the room besides taking screenshots. From my cursory exploration, it can be used almost immediately as it doesn't have a steep learning curve. Sure, it doesn't have many features, but many feature-filled online collaboration tools are also really difficult to use!
Oh yes, forgot to mention, the import a URL/website function didn't work for me at all.
Hat tip to Learn Tech News by the Centre for Learning and Performance Technologies.