How to use Private messages.

You can send private messages to each other. And use it like a private type of chat or email system. How you go about using it. If you are in a post and the other person is in there then at the bottom to the left of their post is some icons.... one has a pm next to it. Click on this and it will open up a message box for you to send a PM to this person. Or you can go to the Memberlist at the top of your screen and then click on the pm there.
I am not sure how much you can store in your inbox on it so keep them tidy so you don't miss any messages.