add_role

<?php add_role( $role, $display_name, $capabilities ); ?> 

<?php
$result = add_role('basic_contributor', 'Basic Contributor', array(
    'read' => true, // True allows that capability
    'edit_posts' => true,
    'delete_posts' => false, // Use false to explicitly deny
));
if (null !== $result) {
    echo 'Yay!  New role created!';
} else {
    echo 'Oh... the basic_contributor role already exists.';
}
?>