What access does the additional cardholder have?
An additional cardholder has all the same access to your credit card account as you do. They will have a card provided to them and they will be able make transactions just as you are able to.
If you have multiple accounts with Beyond Bank and you have an additional cardholder attached to your credit card, the additional cardholder will not be able to access the accounts to which the credit card is not attached, unless you provide express permission for them to do so by making them a signatory.
https://www.beyondbank.com.au/need-help/what-access-does-an-additional-cardholder-have