Anyone with the app can sign in and when done setting up, sign out. If the device hasn't signed in, the music on it (This Phone) can be played or, if the user's account is still active, starting Pandora, as an example, will continue playing their music. With a new phone, as long as the app has been installed and they sign in using the e-mail and password, nothing more should be needed unless the app needs to be updated, AFAIK.