A User drop down is included within the Write / View Report page of users for whom view all users' clients / reports has been activated.

Once activated, the user will be able to select another user with their company or workgroup from the user drop down and search for clients belonging to that user.  They can then edit the selected user's existing client(s) / report(s), or write a new report for an existing or new client of that user in the usual way.