What us difference between distributed computing and workgroup computing?
Workgroup is Microsoft's term for peer-to-peer local area network. Computers running Microsoft operating systems in the same workgroup may share files, printers, or Internet connection. 
They call small local area networks such as home or hospital as 'workgroup'. But it's performance is best with fewer number of computers. You can share resources like printers, scanner, etc,. between computers connected on workgroup. Also, it is not safe for sensitive information and there is no centralized management of the resources. 

Whereas, distributed computing is the field of study in computer science.  E.g. distributed database, aircraft control systems, routing algorithms, grid computing could computing etc., 

Now, I ask you one question: Can workgroup be that part of distributed computing or vice verse or both are possible? I hope this question will help you dig out more information and learn little bit more about workgroup and distributed computing.