To add a comment to a task, the first thing that needs to be done is to expand the task into a more detailed view. This can be done by clicking on the small, grey arrow to the left of the task title:
The ‘Comments’ field will now be visible:
Click on it and type in your comment. When you are done, click the ‘Add comment’ button:
A notification and email will be sent to the task setter and the other assignees.