There's an open investigation with regards to adding users. Our product engineers are aware of this issue, and are now working hard to fix this the soonest.
I recommend giving our Customer Care Team a call to add your account details to the list of affected users. This way, they can pull up your account in a secure environment and sign you up to receive email updates as soon as they become available.
To contact QuickBooks Online Support:
Click the Help icon.
Hit Contact us.
Enter "invite users" in the description, then click Let's talk.
Choose Get a callback.
To ensure we address your concern on time, check out our support hours.
Feel free to go back to this thread if have further questions. I'll always have your back.