This paper reports on a study of text-based communication in a Web-based groupware system, DHS. The system supports annotation of a set of shared documents, where the annotations are gathered in a common dialogue space for each document. Since the system does not support threading of messages, it is of interest to explore how participants use the referential space to connect to each other's contributions. The analysis reveals that the evolving dialogues are characterized by a dual discourse context of documents and comments. The modes of linking are conversational and often implicit, which may lead to coherence problems. We present a visualization tool designed to explore interactivity and referencing strategies in these document-centered dialogues, and discuss implications of our findings for the design of computer-mediated communication systems.