I have been interesting in how the private messaging inside wave works. More specifically what can the participants how has the private message see. I don’t know why I have never tested it out.
I recorded a small video of how this works. (Sorry about the background noise)
The conclusion is that the participant in the private wave can only see the content inside the private wave. He cannot see it is a part of a larger conversation. That is also what you need. The participant in the private wave must not be able to see, waves he has not been added to.
This also makes sense from a productivity perspective. He should not see everything else; he does not need to comment on that.
So when you create a private wave, you need to remember that the user you are added cannot see the rest of the conversation. The content the new participant, should comment on needs to be copied into the private Wave.