Hi Juri Zeisler,
You may need to get a look on how the UserGroupCallback is implemented in your environment.
If you use the default User Group Callback from JBPM 5.4 Installer, it will look for valid user and groups from
\jbpm-installer\jboss-as-7.1.1.Final\standalone\configuration\users.properties
and
\jbpm-installer\jboss-as-7.1.1.Final\standalone\configuration\roles.properties
But this is just a default implementation,
You can even make your own User Group Callback implementation to get valid user from somewhere else,
Take a look at the documentation to get more info:
http://docs.jboss.org/jbpm/v5.4/userguide/ch.human-tasks.html#d0e5419
Regards,
Thomas Setiabudi