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You must be using at least iOS 11 to use these features, so make sure your phone is up to date. You’ll usually find this app on the home screen. It’s at the top of the screen.  It’s the blue button at the bottom of the info screen. It’s the third option in the list. It’s the large blue button. A “New Message” window will open. You can type their name, phone number, or email address—as long as this person is one of your contacts, your iPhone should be able to find them. If you want to include a message, tap the box below the “To” field, and then start typing. It’s the blue circle with a white arrow inside. This sends an invitation to the recipient. A confirmation message will appear. When your family member receives the message, they can tap the link in the message to confirm the invitation. Once confirmed, they’ll be sharing your iCloud Storage space with you (and anyone else you add to the family).

Summary:
Update your iPhone or iPad to the latest version. Open your iPhone or iPad’s Settings . Tap your name. Tap Set Up Family Sharing. Tap Get Started. Tap iCloud Storage. Tap Invite Family Members. Type the contact with whom you want to share the space. Type a message (optional). Tap the Send button. Tap Close.