I haven't tried this in Dynamo, but my trusty RevitPythonShell thinks this should work:
worksetTable = doc.GetWorksetTable()
activeId = worksetTable.GetActiveWorksetId()
workset = worksetTable.GetWorkset(activeId)
this is based on the example from the Revit 2014 API document in the SDK...
The output:
>>> workset
<Autodesk.Revit.DB.Workset object at 0x000000000000002E [Autodesk.Revit.DB.Workset]>
Based on your example, you probably want to do this at the end:
OUT = activeId