You join a Hypothesis group by visiting a link that’s been shared with you—while logged into your Hypothesis account (sign up here). Once you’ve joined a group, that same link will serve as the group home page. At the group home page, you can see a list of group members with links to their annotations and links to the documents annotated by the group. You can also link away to a stream of annotations created by group members across various documents.
NOTE: once you link from the group home page to a document to be annotated, you’ll need to 1) activate Hypothesis somehow (Chrome extension, bookmarklet, etc.) and 2) toggle the scope selector in the upper left of the Hypothesis sidebar to the appropriate group from the “public” group—once you’ve done this once, Hypothesis will remember your preference for the duration of the session.
Here’s a video tutorial that walks through how to join a group you’ve been invited to:
And here’s a step-by-step slideshow that does the same: