Travis - there are several ways to accomplish this, some easier than others.
General approaches:
1>Email and/or File Encryption
2>Transparent (SAAS) Messaging encryption
3>Messaging Portal
Unless the Doc you are messaging has propeller-head tendencies, #1 is out. It is great stuff, but requires someone to manage the encryption or shared/private keys. There are SAAS providers that offer on-demand encryption, but you would both have to subscribe to the service.
#2 is an easier-to-use approach, and may allow a non-user to go to a site to open the email, but you will have costs associated.
#3 is another option, and happens to included in UpDox if you are using them already. The message is routed to the other Doc, and you aren't charged anything for the message/file transfer.
Unless you are a big enough shop to pay someone to manage encryption tech and keys, I'd recommend options #2 or #3.
