To use MultiChain's (and bitcoin's) ECDSA encryption scheme to encode a message for someone's public key, you can read this bitcointalk thread. But just as easy to use some other encryption scheme such as RSA, and for each user to publish their RSA public key in the blockchain, in a transaction they sign in the regular way.
There isn't currently a way to store private information in a node, but if it's just on that node, you can of course use any other kind of local database instead.
