A user drop down menu is included within the Write / View Report page of users for whom the share clients / reports feature has been activated.

Once activated, you will be able to select another user with your company or workgroup from the drop down menu and the clients belonging to that user will be displayed in the client list below. You can then edit their existing client(s) / report(s), or write a new report for an existing or new client of that user in the usual way.