Let New Users Pick an Account Type

// Role ID => label shown on the form. The first one is the default.
$wpide_signup_roles = array(
    'subscriber'  => 'Reader',
    'contributor' => 'Writer',
);

$wpide_safe_roles = function () use ( $wpide_signup_roles ) {
    $danger = array( 'manage_options', 'edit_users', 'promote_users', 'activate_plugins', 'edit_others_posts', 'unfiltered_html' );
    $safe   = array();

    foreach ( $wpide_signup_roles as $role => $label ) {
        $object = get_role( $role );

        if ( $object && ! array_intersect( $danger, array_keys( array_filter( $object->capabilities ) ) ) ) {
            $safe[ $role ] = $label;
        }
    }

    return $safe;
};

add_action( 'register_form', function () use ( $wpide_safe_roles ) {
    $roles   = $wpide_safe_roles();
    $current = isset( $_POST['wpide_signup_role'] ) ? sanitize_key( $_POST['wpide_signup_role'] ) : key( $roles );

    if ( count( $roles ) < 2 ) {
        return;
    }

    echo '<fieldset style="margin-bottom:16px"><legend>I am signing up as</legend>';

    foreach ( $roles as $role => $label ) {
        printf( '<label style="display:block"><input type="radio" name="wpide_signup_role" value="%1$s" %2$s> %3$s</label>', esc_attr( $role ), checked( $current, $role, false ), esc_html( $label ) );
    }

    echo '</fieldset>';
} );

add_action( 'register_new_user', function ( $user_id ) use ( $wpide_safe_roles ) {
    $role = isset( $_POST['wpide_signup_role'] ) ? sanitize_key( $_POST['wpide_signup_role'] ) : '';

    if ( $role && array_key_exists( $role, $wpide_safe_roles() ) ) {
        ( new WP_User( $user_id ) )->set_role( $role );
    }
} );