Simply put Ninjam is a way for people to jam with each other across the internet. It is free and open source software. It’s official website is here:
https://www.cockos.com/ninjam/

If you are tech savvy and have a server you can create your own place to jam with whoever you want around the world. Check out their website! This tutorial is for keeping it simple for people who just want to get on and jam quickly, with no major complicated technical stuff.

The two easiest ways

are either by using the DAW Reaper, and the free standalone and VST software Jamtaba.

Using Reaper you simply add the Ninjam plugin to the master track, and you are all set to go.

Inside the plugin if you click connect you will see a list of open public servers you can jump into.

You can do this with Jamtaba as well. Jamtaba is both a standalone App, and it is also a VST audio plugin. Using it as a plugin inside a DAW of your choice, can give you more options. However, it can also be used on its own. Simply scroll to pick a room and click enter.

If you have a friend with a private server all you need is the ip or url address, and click the private server menu.