Hi Arun,
Maybe Try, You remove the dependency start.human.task from the start.demo target in build.xml, it raises start.human.task service, this target also runs a class located in {jbpm-installer}/task-service/src/org/jbpm/DemoTaskService.java, in which you have to comment out the lines of adding users and groups and not have problems with creating users and groups, and has already created the install.jbpm5.
The start.human.task target must be run separately to init server minaTask.
Wilman