ThruBook is a collaborative work platform based on ebook documents, supporting synchronized group work sessions. Using this platform, users can participate in a group session based on a same ebook, and share group actions synchronously. In this paper, we consider annotations as all kinds of additional data on ebooks, including bookmarks, memos, and figures. Also, model shared actions such as sharing/navigating these annotations. As a result, we could model all collaborative actions as annotations, and propose a collaborative work model as communicating annotation objects between session participants. This approach implemented in ThruBook platform could easily model shared actions in collaborative systems. The collaborative work platform implemented in ThruBook can be applied to other types of collaborative work systems.