I'd like to get you in touch with the correct group to discuss the licensing options available to you.
The QuickBooks sales team will be able to provide information on adding more licenses to this version of QuickBooks. Their phone number is located at the top of the following website on the right-hand side of the screen: https://quickbooks.intuit.com/.
If you have any additional questions, please feel free to let me know.
Why 2015 which is already discontinued for services? It's time to upgrade to the latest version of 2018!
The maximum number of simultaneous users (users accessing the file at the same time) for QuickBooks Pro is three users and Premier Edition is five users. A server does not count as a user if no one works on QuickBooks on the server itself. For more info, click here.