How do I add referees under existing matters?

In the left menu bar, click on the heading 'Docs' Then, select 'Matters'

To edit an existing matter, click the matter's name from the list of matters.

Under the matter's name, select the heading 'Referees' Then click 'Add referees'

Here you can select the referees relating to this matter.

Once you have selected all referees relating to this matter, click 'Add referees' at the bottom right of the popup.