WordPress does not really have a native feature that allows you create different user access levels for one particular piece of content, but there are multiple easy workarounds to achieve this.
If you need to add an additional password to protect a page or post, or perhaps you need to use many passwords, there is a great tool that makes it really easy to do.
Option 1: Multiple Post Passwords plugin
The Multiple Post Passwords plugin by Andreas Münch “lets you set multiple passwords for your password protected posts and pages” and, just as an extra, this plugin also allows you to set an expiry for passwords to expire after a certain time.
“Very good plugin. It allows multiple passwords for each site. Works fine.” – elorien on WordPress.org
The plugin supports multiple passwords (literally unlimited), and works on pretty much any post type imaginable, including custom posts, all posts, pages, and other formats.
This plugin allows you to password protect specific pages and posts on your site, each with its own set of passwords. This means that if you need access for multiple users, you can create different passwords for them without having them all share one single password.