Hypothesis is a new effort to implement an old idea: A conversation layer over the entire web that works everywhere, without needing implementation by any underlying site.
Using annotation, this enables sentence-level note taking or critique on top of classroom reading, news, blogs, scientific articles, books, terms of service, ballot initiatives, legislation and more. Everything built is guided by Hypothesis' principles, in particular that it be free, open, neutral, and lasting to name a few.
The group feature is great for groups of students or researchers working together. Features include highlighting text, annotating specific text, or leaving page notes which are not anchored to any text. I see there are some independent mobile apps available too.
There is a good video explanation of what you can do at Introduction to Web Annotation with Hypothesis
In this video, you will learn the types of annotations you can create with Hypothesis, how to sign up, and how to create groups to annotate with your student...