This is a tip that many of you probably already know, but was summarized nicely by Brian Swetland:
How to send a “personal” message on Google+.
- Type message into the share box
- Modify the list of people you’re sharing with to contain just one person (for example, my friend Joe in the example below)
- 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.