Spotify is now available on all Phorus audio products, but the way it works is different from the other streaming services. Instead of Spotify living in the Phorus app, the Phorus product itself communicates directly with Spotify. To enable this feature, simply download the latest version of a Phorus app, and allow it to update your Phorus product.
Once you're all updated and ready to go, open up your Spotify app, start playing something, and then, on the Now Playing screen, press the Spotify Connect button, which is the little speaker icon in the bottom right of the screen:
You'll then be able to see all the Spotify enabled devices on the network (including your Phorus and other Play-Fi products). The selected products are now tied to your Spotify Premium account, and as long as the product can communicate with the Internet, you can play to it from your Spotify app.