You certainly can have multiple businesses under your login however each company would require a separate subscription. Once you sign them up you will see a list of active companies when you first log in and you can choose which one to view/work on.
Resolution for 'additional business issue: 'Can I add additional business' available: Yes.
The additional business 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 additional business issue yourself by looking for a resolution described in the article. If it is a complex additional business issue or you are unable to solve the issue, you may contact us by clicking here or by using other additional business support options.
For more information about resolving issues related to additional business in Quickbooks/QBO or to get support for the issue 'Can I add additional business', click the link below: