how do you configure the plugin to send pm to others on the site and the others reply?
You need to have plugin Private messages installed. Go to unistalled plugins and install it.
Then go to Configuration and set what you need.
Don't forget to test your PM mail notification.
If anythiing is not clear, just ask.
You are welcome. I am sorry you needed to ask here but I am tired to answer the same questions again and again. If anybody ask the same thing, I just point them here. Thank for your understanding.
Oh, and this way in fact I am testing forum plugin :)