When you reply to a message, if the reply settings are set to "Group & Sender" then the reply goes to both. Although you can click on the "Private" button on the far right and it changes the reply button to "Reply to Sender."
I would like to suggest simplifying the process by eliminating the private button and having 2 reply buttons, "Reply to Group & Sender" and "Reply to Sender," hot when the settings are set to"Group and Sender." By having the option all the time, it would (okay might) make the sender think "Do I need to send this to the group, or just to the sender?" Overall it could reduce the number of messages to the entire group, plus allow people to more easily communicate things which do not necessarily need to go to everyone.
It would also be nice if the sending of private messages were recorded in the activity log. Not the message itself, but just the fact that it was sent.