How does a shared vault in password managers such as 1Password work?

The password manager 1Password has a feature where multiple accounts in a group ("family") can share login information with each other.

From my understanding, a password manager is never supposed to know my passwords because they are encrypted with my master password before being sent "to the cloud".

How then can I decrypt / see the password that a family member shares with me through the Shared Vault without 1Password decrypting it?

If all passwords are encrypted with my private master password, how can it be possible that another user can decrypt it without me or the password manager knowing the master password of the other person?