You join a hypothes.is group by visiting a link that's been shared with you--while logged into your hypothes.is 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 hypothes.is somehow (Chrome extension, bookmarklet, etc.) and 2) toggle the scope selector in the upper left of the hypothes.is sidebar to the appropriate group from the "public" group--once you've done this once, hypothes.is 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: