Hi !
This is my turn to reply to you.
I have implemented my own userGroupCallBack with ldap. And for this part it works more or less. When it checks a user existence, it automatically added him (or the group) in the database.
But, when the user already exists in the database it still tries to insert it and rise a MySQL duplicate primary key error. But this error desn't lock the process.
So you don't need to load users and groups in the table.
Regards,
Sandra