Sharing from Android phones to iPhones is the same as sharing from Android to Android.
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Google Wallet: Simply forward the email you received and ask them to click on "Download Membership Card > Add to Google Wallet" the same way you did (Google does not have a sharing option).
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Wallet Union:
Open the Wallet Union app
Tap on your digital card
Tap on the three dots icon at the top right side of the screen
Tap on "SHARE CARD"
We recommend sharing via email, or Bluetooth
Images below as a reference!
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Wallet Passes (no longer supported):
Open the digital card on your Android (the installed card).
Go to the back of the card by tapping on the "i" button at the bottom right side of the card.
Tap on the "Share Pass" button.
Use one of the sharing options displayed on your phone (we suggest you use the Email option).
All set!