Secure Secret Sharing to a Distributed Key
From what I understand, you want to share a secret to B in such a way that every single member of B is required to reconstruct the secret. In that case just use a threshold secret sharing scheme where the threshold is the number of players in B. This ensures that no subset of players in B can reconstruct the secret.
I’m not convinced you need encryption here (as secret sharing is already information-theoretically secure. But if you do, then encrypting under public key and sharing to B such that the threshold is equal to the number of shares should suffice.