As noted, the only way to do this while honoring Bitwarden’s commitment never to have access to unencrypted data is to use public key cryptography. Only a device which has access to the unencrypted data and to the recipient’s public key can encrypt that data (or, more practically, a randomly-generated key which is then used to encrypt the data) so the recipient can decrypt it with his or her private key.
That cannot happen on the server, because that would mean the server would have access to the unencrypted data. It can only happen on the sharer’s device; and it has to happen when he or she sets up the future sharing, because there is no guarantee that device, or any device which already has access to the unencrypted data, will be available later.
This makes it impossible to share with an individual who does not already have an account (hence a pass phrase, hence a public/private key pair). The recipient’s public key has to exist at the time the data is shared, regardless of whether that data is to be made available to the recipient now or in the future.
I don’t see any way around that without compromising the principle that Bitwarden’s servers will never have access to unencrypted data.