Hi,
when designing a process you have to specify actor(s) or group(s) of the particular human task. User who is actor or member of the group is able to view or complete these tasks for example in jbpm console or in other task client.
Users for Guvnor are created in jbpm-installer/auth, if you use jbpm installer. But you have to add these users to human task service too in jbpm-installer/task-service/resources/org/jbpm.