Mozzy is the companion app for members of a band on Plus or Ultra — see Account if you're not sure which plan your band is on.
Search for Mozzy on the App Store (iPhone) or Google Play (Android). A QR code that takes you straight there is on its way too — for now, search by name.
You'll sign in with the same email and Backstage password you already use on the website — nothing new to set up. See Logging in for the actual sign-in flow, and Using multiple accounts on one device if you play in more than one Mozivio band.