An instant messaging system generates an instant messaging application user interface display object enabling the user to select one or more buddies, or a predefined group from their buddy list, and then to select a file to share with the selected buddies or group. The instant messaging system then controls sharing of the file within the selected buddies or group, by allowing only one of the sharing users to check out and modify the shared document at a given time. The instant messaging system user interface displayed to each user also displays the list of files that are shared with that user. The instant messaging system user interface further provides a current status for each shared file, including an indication of one of the sharing users that currently has the shared file checked out.