How to send a "personal" message on Google+

1. Type message into the share box
2. Modify the list of people you're sharing with to contain just one person (for example, my friend Joe in the example below)
3. Push the Share button.

Note that you or your friend can invite more people into the conversation later by @mentioning(or +mentioning) them. Your friend could use the "Share" button to reshare the message -- you can disable sharing with the (v) dropdown in the upper right corner of the message if you want that option to go away.