Dbo User Cannot Add New Users

In SQL 2005, a user in the db_owner role cannot add new users to his database.
The error message says (generic name inserted by me):
Create failed for User 'ourdomain/someuser'.
'ourdomain/someuser' is not a valid login or you do not have permission. (Microsoft SQL Server, Error: 15007)

The login is valid. I can add it easily since I have sa permissions.

In SQL 2000, a db_owner could add a new user for any existing login, but in SQL 2005, the "browse for objects" dialog box for adding a new user only shows yourself.

How can I allow database owners to add new users whenever they want without also allowing them full control over all logins? In other words, I do not want to give them the securityadmin server role.

Thanks.
-David