Welcome to the QuickBooks Community. It's important you get the help you need so you can continue to focus on your business with peace of mind. Rest assured, I'll be happy to share more info so you're on the right track.
In order for you to accept an invite from the US QBO, the admin of that account would have to send an invite to you via email. Since US QBO and QB Canada are separate, you can still accept the invite using the link in the invitation email. Let me know if this info helps you. You're more than welcome to ask other questions in the comments below. I've got your back.