It doesn't look like this works in my case.
I need owner not actor id. I my situation, I only identify a group id then some member of that group claims the task. I want to know which group member claimed it. I am guessing that, with an exit action or script node, there is no way to access the task (which would allow me to identify the owner). Here is my on exit action which didn't work (owner is null):
#
String owner = (String) ((WorkItemNodeInstance)kcontext.getNodeInstance()).getWorkItem().getParameter("ActorId");
System.out.println("Owner is: " + owner);
kcontext.setVariable("manager",owner);
However I think I can handle this OK on the application side i.e. the task owner will identify theirself to the process instance.