You may send an email to firstname.lastname@example.org together with the name and contact number of the person you wish to be listed as an authorised account user.
In doing so, you're giving an authority to the person to contact TPG on your behalf and authorise TPG to discuss account related information such as; billing, contract, technical issues and account changes except for account cancellation.
You can simply send the request via email and send it to email@example.com.
Should you need further assistance, feel free to search community for help or chat with us at https://www.tpg.com.au/support/contact/ and we'd be glad to assist you.
I would like to add Grant xxxxx my IT support to this account.