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.