The mobile app runs on iPhone, iPad and Android, and it is a full Grape rather than a companion viewer.
iPhone and iPad
The iOS app is in beta through TestFlight. Install TestFlight from the App Store, then join through the TestFlight link on the download page. On iPad the app rotates freely, so it works in landscape and with a keyboard.
Android
The Android app is on Google Play. Install it from the store listing linked on the download page like any other app.
The first open
No account, no setup. Grape creates a local library on the phone, adds a couple of sample notes, and shows a short welcome tour of the main features. Permissions are only requested when a feature needs them, like the microphone the first time you record.
Using it with the desktop app
Notes stay on the phone unless you turn on sync, which keeps the phone and the desktop app on the same library with a Pro subscription.