Getting a "User or role '[DomainUsersAccess]' does not exist in this database." error

I've got a SQL Server 2008 R2 Express database. We're working on a WPF app, that's going to use this database, which will be installed on each users' desktop/laptop. We want it so that access to the SQL Express database will be handled by Windows authentication, so if a domain user is logged in he/she can access the data. The database is named ASI. The plan is to create a new role, assign the db_datareader and db_datawriter roles to this new role, associate a user in SQL for all domain users and associate the new, custom role with all domain users. We've got a group in our AD named "Domain Users", and that's what I was going to associate with the user in SQL. However, I'm getting an error trying to do this, I don't have to give a lot of lines to illustrate where the error occurs. Here's the SQL:
USE ASI GO CREATE ROLE [DomainUsersAccess] --give my new role appropriate permission to ALL tables EXEC sp_addrolemember N'db_d ...