How to invalidate decryption key in multi user model?
Sure.  Encrypt John's private key under Adam, Emily, and Scott's public key.  When you revoke Emily and change John's keypair, encrypt John's new private key under Adam and Scott's public key.  If Adam or Scott need access, they can decrypt.  After the key change they will still have access and still be able to decrypt it, same as they were able to before.