As collaborative computer systems are evolving, the use of spatial, three-dimensional interfaces for multiplayer games, groupware systems, and multi-user chat systems, for example, is increasing rapidly. This paper provides a theoretical underpinning for understanding the relevance of user embodiments and copresence within such three-dimensional collaborative computer interfaces. Firstly, the issue of embodiment is traced back through its origins in philosophy and psychology literature, and theories are identified, potentially helpful in understanding key issues concerning user embodiments in collaborative virtual environments. A hybrid avatar/agent model to achieve permanent user embodiments in such environments is discussed. Since copresence of other users within such environments has been shown to be an important factor for the experience of presence, a prototype embodied conversational agent has been designed to simulate copresence. A series of controlled experiments involving the prototype agent is discussed, highlighting the effects of simulated copresence on users’ experience of presence. Results suggest that, despite its shortcomings, the prototype agent does seem to have increased participants’ experience of presence. Evidence was found that even limited copresence as provided by the current prototype agent is sufficient to help users feel presence in the environment. The results seem to confirm that copresence simulated by agents can complement avatar technology and therefore that a hybrid avatar/agent model can potentially achieve permanent virtual presence of all participants.