If you'd like to ensure that an additional driver is covered, you should list them on your policy. If someone who isn't listed is driving your car and an incident occurs, you may have to pay an additional excess, or you may have your claim denied.
Learner drivers are an exception. If the person supervising them is listed on the policy, the learner driver doesn't need to be. An additional age or inexperienced driver excess may apply though, in the event of a claim.
People with international licenses also need to be listed as an additional driver if they’re going to be driving your car regularly. As long as they're driving legally - that is, their license is recognised by the relevant state or territory authority - they'll be covered.