taskFormData = task.getExecution().getProcessEngineServices().getFormService().getTaskFormData(task.getId());
reviewers = task.getExecution().getProcessEngineServices().getIdentityService().createUserQuery().memberOfGroup("reviewers").list();

formFields = taskFormData.getFormFields();
if (formFields!=null){
   for(field in formFields){
       if (field.getId() == "reviewer"){
           items = field.getType().getValues();
           for(reviewer in reviewers){
               items.put(reviewer.getId(),reviewer.getFirstName()+" "+reviewer.getLastName());
           }
       }
   }
}