Family plan
How do I add other phones to my iCloud backup plan?
iPhone 12
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If they are owned by you, sign into them using your Apple account information.
If they are owned by others, invite them to Family sharing and make sure you have a family level subscription to iCloud.
Click on this link to learn how to set up Family Sharing --> Set up Family Sharing - Apple Support
Read the section: "Invite people to join your family"
and at the very bottom:
"Remember, you can only join one family at a time, and you can only switch to a different family group once per year."
After you send the invitation, you can check its status under the person's name in Settings > iCloud > Family on your iOS device. On your Mac, go to Apple menu > System Preferences, click iCloud, and select Manage Family. Then select that person's name to see the status of the invitation.
If you need to resend the invitation, select Resend Invitation."