Could you please explain me why this inconsistency occurs? When I create a user without any group assignment it shows it is a member of camunda BPM Administrators at the Welcome Page. On the other hand in the Camunda Admin page there is no group at all this user is member of. Moreover I can do any administrative tasks with this user.
Answering your question, no the initial user is created when application first runs and initialize a database. The user is root. The user ja has been created via UI.
The problem I had was trivial though, the authorization plugin was not enabled shame on me. So when it is not enabled any user apparently is implicitly member of camunda-admin group.