Some more info. I ran the SQL script attached to SQL Express using my Windows account. My account has the SysAdmin role assigned to it, so I would think it would be able to create the DomainUsersAccess role. And yet, it isn't in my instance of SQL 2008 R2 Express. Why is that? I'm sorry, I was wrong. The DomainUsersAccess role does exist in the ASI database in 2008 R2 Express.

It doesn't look to me as though roles like db_datareader or db_datawriter have been added to the DomainUsersAccess role in the ASI database, though.