At our company, we use our own solution: "nCryptedCloud" (http://ncryptedcloud.com), client-side encryption software that secures your files before they go into the cloud (Dropbox). We don't keep the data, just the knowledge how to open that data (key). We don't even have a link between the file and the key, the file contains the key's information. This way, using dropbox shared folder, you can manage a secure sharing experience that allows you to revoke access to anyone in the future. It's free. Check it out and let me know if you have any question :)