Hi Cedar,
Thank you for the post. I understand you have an MYOB with one company file online and you're looking to add another one. You can definitely add an additional company file online by purchasing another subscription. To purchase the additional subscription, please go to Contact us for help.
For the login details, you can keep things simple by using the same email address to manage both company files under your existing MYOB Account, but if you prefer, you can set up a new MYOB account with a different email address for the new company file.
To let others access the new company file, you can invite other users by adding their email addresses under the 'Users' section in your MYOB account settings. We have a detailed guide on how to add users here, which provides step by step instructions.
Regards,
Sai