This app requires using a Google Calendar account.
Set it up on your device
Please add a "Google" account in the "Accounts" section of your device's "Settings" app.
- Open the device's Settings app.
- Tap [Passwords & Accounts].
- Under [Accounts], tap [Add Account].
- Select the type of account to add.
- Follow the on-screen instructions.
Share Google Calendar
Please refer to the instructions on the Google Calendar sharing settings page.
https://support.google.com/calendar/answer/37082?hl=en
It's simple, so please give it a try!