You encode the cleartext with a single master key. Then you encrypt the master key with multiple keys and store it together with the encrypted cleartext. This

You encode the cleartext with a single master key. Then you encrypt the master key with multiple keys and store it together with the encrypted cleartext. This will not take much more space as only the master key is encrypted multiple times. Now anybody who has one of the keys can decrypt the master key and subsequently decrypt the cleartext.