You can try to cast it to HumanTaskNodeInstance if you're sure it is a HumanTask and get a work item out of it. Next, access parameters of work item to get actor id and group id.
String actorId = ((HumanTaskNodeInstance) nodeInstance).getWorkItem().getParameter("ActorId");
String groupId = ((HumanTaskNodeInstance) nodeInstance).getWorkItem().getParameter("GroupId");
HTH