function my_filter( $content ) {

    $categories = array(
        'news',
        'opinions',
        'sports',
    );

    if ( is_category( $categories ) ) {
        if ( is_logged_in() ) {
            return $content;
        } else {
            $content = '<p>Sorry, this post is only available to members</p>';
            return $content;
        }
    } else {
        return $content;
    }
}
add_filter( 'the_content', 'my_filter' );