I suspect not, but haven’t heard back one way or the other, so can’t guarantee. Each private message seems to be just another separate Discourse thread in all other functional aspects, except it’s visibility limited to the original poster, the specified recipient, and any other @ mentioned.
Maybe @tiwane can provide a more definitive answer?
EDIT: oh wait, maybe you mean previous posts within the same private message thread? Yes, I believe those are visible to newly invited participants. Just not other, separate PM threads. I think.