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This is a neat service. Once you install the browser extension, you can click the toolbar icon to get a menu on any page you visit:
From there, you can highlight text or add comments for anything on the page. Where this gets powerful for classrooms is when you start creating groups. Any highlight or annotation can be public or shared with a group. In the group, only other group members will see the markup and can comment or add their own.
Unfortunately, you and your students will need an account and you can’t use any sort of single-sign on service (Google/Microsoft logins for example).
Once you have an account and are signed in, you can create a group. Each group has an invite link.
This gives you an easy way to add students to the groups.