QB Issue Resolution:

The 2015 program will not be retired/sunsetted/discontinued until 5/31/18. If you going to purchase additional licenses you may want to upgrade first.

Resolution for Issue 'Increase Number of Users' available: Yes (Solved).
Source: Intuit Community forum.

Increase Number of Users: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for additional licenses
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Error Recovery for additional licenses [Canada]
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Additional Support Options: