I have seen several people sending out a group message that might have been meant as a private message. What is the difference?
A group message is sent out to all the members of the co-op. You can make a group message by clicking the button on the handshake tab of the co-op menu or by clicking the button group message in the message menu (reached by clicking the envelope in your farm menu)
A private message is sent to one particular player. You can make a private message by clicking on that person in either the member tab (blue person) or chat tab (green speech balloon). You can also go to the message menu and type in the name or go to the rankings, search for the person, click on their name and click the message button.