When a user is invited to QuickBooks and cannot sign in, if they have a user ID in the invitation, have them click on it so, it will route them to log in. If no user ID, they have a link to create one on the email where the invite is sent. Once a user ID is created, they can now sign in.
However, if done with the steps above, to isolate the issue, have the user login using a private browser. There are times that the browser is full of frequently-accessed page resources that cause QuickBooks Online (QBO) to act unusual behavior.
Here are the keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once open, have them log back in and check to see if it works. If they are, then their main browser may have too much historical data from previously visited websites which causes this kind of behavior. To ensure this doesn't happen again, it's a good idea to clear the cache of the browser they're using. The following article contains additional insight, as well as steps for the supported browsers: Clear cache and cookies to fix issues when using QuickBooks Online.
But if the result is the same, I recommend contacting our Customer Care Team. They have tools like screen-sharing that will help check the root cause and find a permanent solution to this issue.
To reach them, click the ? Help button at the top-right corner and select Contact Us to talk with a live agent. Ensure to review their support hours so, you'll know when agents are available.
If there's anything else I can do for you, let me know by commenting below. I'd be around to assist. Take care.