I've received a report that some users are getting the "Please enter a valid phone number including area code" error when a user is trying to accept the invite.
Our Product Team has identified the root cause of this issue and is working to implement a fix as quickly as possible. I suggest contacting our QuickBooks Online Care Team so they can add your account to the investigation (INV-39519). This way, you'll able to receive updates about the issue via email.
For now, you can follow the workaround below that they've provided:
For QuickBooks Online Accountant (QBOA):
Get the accountant to set up their Firm manually on the QuickBooks website before accepting the invite (a fresh invite will need to be sent after the firm is created).
For Small Business (SMB):
Get the user who is trying to accept the invite to go to accounts.intuit.com and create a new sign-in (a fresh invite will need to be sent after the login is created) leaving the phone number field blank.