It simply registers the users and groups, so stores their id in the database tables. These tables are then used at runtime to validate the users and groups that tasks are assigned to.
For user/group management in the console, we don't want to maintain this relationship as part of the human task service. So we simply use the normal user authentication / authorization facilities provided by JBossAS (using a users.properties and roles.properties file) for the jbpm-console.
Once we have the callback interface implemented, it will be trivial to register your own implementation and load users / groups on demand etc. You can already change the implementation and for example load the groups of a user on demand, but that would require you to modify the jbpm-console code at this point. Once the callback is there, it will be much easier. I suggest you start pushing tsurdilo on irc.codehaus.org to hurry up his implementation ;)
Note that the documentation on the human task service and the console has been extended recently, so it might be worth rereading those chapters, and hopefully they will help you out as well:
http://docs.jboss.org/jbpm/v5.1/userguide/
Kris