I’m not sure Intuit even allows more users now.  They have come out with the Advanced version that starts with 10 users (at a whopping $150/month).  If no one from Intuit answers this, I would recommend that you call Support.

https://quickbooks.intuit.com/accounting/advanced/

Resolution for 'Advanced version issue: 'Now our user limit is 10 people. Do you know how much is the additional fee if we add one more user?' available: Yes.

The Advanced version issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Advanced version issue yourself by looking for a resolution described in the article. If it is a complex Advanced version issue or you are unable to solve the issue, you may contact us by clicking here or by using other Advanced version support options.

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