You cannot. Under the same section where the link that I posted was, there
is another article that applies to your question on roles:
http://msdn2.microsoft.com/en-us/library/ms178375.aspx
Quoted from the above article
"SQL Server Express Edition
When a process attaches to a SQL Server Express Edition database (.mdf
file), the process must have administrative permissions. In general, this
makes SQL Server Express Edition databases impractical for production Web
sites because the ASP.NET process does not (and should not) run with
administrative privileges. Therefore, use SQL Server Express Edition
databases only under the following circumstances:
Use as a test database while developing your Web application. When you are
ready to deploy your application, you can transfer the database from SQL
Server Express Edition to a production instance of SQL Server.
Use if you are running a Web site that can use impersonation and you can
control the privileges of the impersonated user. In practice, this strategy
is practical only if the application is running on a local area network (not
a public Web site).
Store the .mdf file in your site's App_Data folder, because the contents of
the folder will not be returned to direct HTTP requests. You should also map
the .mdf extension to ASP.NET in IIS and to the HttpForbiddenHandler handler
in ASP.NET using the following element in the site's Web.config file:
<httpHandlers>
<add verb="*" path="*.mdf" type="System.Web.HttpForbiddenHandler" />
</httpHandlers>
"