Something that jumped off the list at me: Adding users to an existing private topic opens a wide gamut of privacy concerns.
I would never ever post to or even read a private topic if I knew that another person could be added afterwards.
I agree 100%, the only way I would think of liking it is if only the person who created the topic can add, a notification in the pm was made that xxxx was added, and the newly added cannot look back through the topic.
