Google's long awaited carrier play was announced recently as Project Fi, an MVNO running on Sprint and T-Mobile that will begin as a Nexus 6 exclusive. The official Project Fi app is now live in the Play Store—not that it will do you a lot of good without an invite, which no one seems to have yet.
Project Fi will switch between Sprint and T-Mobile depending on who has the best LTE signal in your location. This cross-network magic is whay the Nexus 6 is the only supported phone at launch. Although, the app can be isntalled on any phone right now, which is completely pointless. It won't even open on a Note 4, and I imagine it's the same elsewhere.
The Project Fi app can be used to activate the service, manage account settings, monitor data usage, and pay your bill. That's pretty much what we saw when we got a look at the app before Fi was official. Go ahead and install the app if you want, but you still need to sign up for an invite. The APK is also on APK Mirror if you want to sideload it.