feathersjs.com is a framework that uses socket.io (or primus) and expressjs under the hood. The first featured tutorial on their homepage is a chatroom, so it must be a fast way to add one. "See just how easy it is to create a real-time app from scratch, complete with authentication."

I have had a very good experience with Pusher before , it minimized all my engineering efforts on developing the feature , you could try their recently released chatkit as well . As option goes you could go with applozic/ sendbird as well .

Use firebase for chat and you can easily integrate it with mobile app as well. Alok Deshwal

You might like Tawk, it's free, lightweight, easy to integrate (similar to Google Analytics). It provides a dashboard where you can custom the interface, view the statistics, manage the chat conversations, or add more assistants who can help you talk with your sellers, etc.

Where I work, we use Intercom. It took a couple of hours to integrate, after reading documentation.

We've used Olark as well, but we liked Intercom better

If you need something fast you can always integrate a 3rd party option. Olark as an API you can use, and docs for their Javascript API. And Chatra looks newer, if you want something fresh.

I'd double check to see how the 3rd party services store data if you're building a private or HIPPA compliant type application (since you mentioned 'classified').

Or you can use an open source software like this one called Hummingbird. It's pitched as an Olark alternative.

Or socket.io has a good tutorial on making a simple chat app with Node if you're looking for a scratch option.