Well, there it says that Users is not mapped.
If you want to query the users from task database (https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/task/User.java)
you can make "From User"
and when creating the EntityManagerFactory, the persistence unit name is "org.jbpm.task"
Jbpm uses two persistence units (you can check it in file persistence.xml, it should look like this https://github.com/calcacuervo/JBPM5-Samples/blob/master/human-tasks/src/test/resources/META-INF/persistence.xml) One is for human tasks persistence, and the other for jbpm runtime entities.
Hope this helps,
Demian