1Password has a pretty good white paper explaining their security design (PDF behind the link): https://1passwordstatic.com/files/security/1password-white-p.... The parts "How Vault Items Are Secured" and "How Vaults Are Securely Shared" go into sharing passwords in a vault.
For the record, Bitwarden's white paper is a good read as well. Available at https://bitwarden.com/help/article/bitwarden-security-white-....
(edit: fixed typos)