The steps in designing and building a real-time features are discussed in detail in this book called BEANS (Bootstrap ExpressJS AngularJS NodeJS SocketIO).
You can purchase the book at Amazon.com
How to write real-time features in JavaScript. Create your own Chat Messenger the easy way using NodeJS.
There's a lot more you could add to this instant messaging app. For instance, maybe the server could keep a recent history of messages for the benefit of new users joining the app.
Writing AngularJS apps that make use of other libraries is easy once you understand how to wrap them in a service and trigger a notify event to Angular that a model has changed.