how to insert a user in a role at runtime

there are 2 roles in my web appl.one is admin and other is basic.
now i need that if user is in admin role he is redireted to his page and if in basic role he will redirect to basic pages.
i know how to find roles and redirecting them to another page,here user fill the registration form and than admin allow him to see basic pages.new users are added to the basic role by admin at the time of creating user.
so plz tell me how to insert a new user in a role at runtime,

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