I think the idea is to have it on hand already

I personally like to switch music services every year or two when things start to feel stale

First you need to download the TestFlight app from the App Store if you don’t already have it. Then, on the GitHub page, click the link that says “click here and join the group”. After that you will be able to install the app from TestFlight. Later, they will have a stable release in the App Store, and you should be able to switch over to that then, if you like.