Subscription-based music service Napster has joined rivals like Rhapsody on the App Store.Its iPhone app went live yesterday night, having been approved by Apple. The app lets users save songs, albums and playlists to play offline, matching its rivals, while also offering recommendations and automixes.The app is free to download, but requires a Napster plus Mobile Access subscription to use, which costs $10 a month.Last week, Rhapsody boasted that its iPhone app has been much more popular than on-demand rivals like Thumbplay, Rdio and Mog. Now there's one more company in the battle.