Thank you again for the support.
The suggested way seemed a bit complicated, so I made up an easier attempt which at least seemed to work.
In the according User Task, I added some lines of code to the ViewDefaultviewView method completeTask().
This basically copies the String content from the PrincipalList to a field of Business Data I created for this Task (called ListOfPeople = a List of Strings).
// do the work
String[] tmp = new String[getPrincipalList().size()];
for(int i=0;i<getPrincipalList().size();i++)
{
tmp[i]=getPrincipalList().get(i).toString();
}
getPickPeople().getTaskData().setListOfPeople(tmp);
// work done
So what happens is, on clicking the Complete button, the content of the PrincipalList is externalized to Business Data and available at the next step.
#webMethods-BPMS#webMethods#MWS-CAF-Task-Engine