Navigate to the project to which you want to add auditors. Make sure you’re an administrator on the project - you won’t be able to add auditors otherwise. Click the gear icon and select Project Settings.
This will open the Project Settings modal. Under Manage Collaborators, you can mark any collaborators as auditors by checking the box in the Auditor column. You can also add additional collaborators in the text input field.
When you're done adding auditors, click Save.