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 '10 people 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 10 people 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 10 people issue yourself by looking for a resolution described in the article. If it is a complex 10 people issue or you are unable to solve the issue, you may contact us by clicking here or by using other 10 people support options.

For more information about resolving issues related to 10 people in Quickbooks/QBO or to get support for the issue 'Now our user limit is 10 people. Do you know how much is the additional fee if we add one more user?', click the link below: