Described is a system and method by which client programs such as desktop applications can use instant messaging functionality. An application program interface is provided that allows a client program (e.g., an application program) to interact with an instant messaging infrastructure, such as via event calls to initiate conversations, send messages and/or obtain presence information. This enables other programs to create new user experiences that integrate the messaging infrastructure, such as displaying presence, or enabling instant messages to be sent from within an application program. Document collaboration is also facilitated, by allowing users to send instant messages (or the document) to other users, including sending an instant message directly to a user associated with a specific comment or change to the document.