How to assign role for user from ActiveDirectory

Hi all,
In my intranet application, I use ActiveDirectoryMemberShipProvider to authenticate users against Active Directory, and I use SqlRoleProvider for role management. There're so many users in the active directory  and my application only allow some of them to access.
Now my concern is :
    - How to allow only specific of users from AD to access my application.
    - How to manage role for these users.
Thank you very much

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