According to jBPM User Guide, Task Server requires all users and groups to be predefined before trying to assign a task to that user or group. So you need to make sure you add the necessary users and group to the database using the taskSession.addUser(user) and taskSession.addGroup(group) methods.
Definitely, having all user accounts and groups replicated from and kept in sync with primary user management system (database, LDAP, whatever) could be hardly considered a good practice.
I wonder what is the recommended way to integrate task server with an external user repository?
Thanks,
Alex