Oh okk.. yeah you need to modify the build.xml and put the following line.
<java classname="org.jbpm.DemoTaskService" fork="true">
<sysproperty key="jbpm.user.group.mapping" value="file:///<path of the file>/standalone/configuration/roles.properties"/>
<classpath>
<pathelement path="${install.home}/task-service/target"/>
<path refid="classpath.human.task" />
</classpath>
</java>