The expression POG (short for Pack of Green) comes from Diablo Bardo Safaris. In Diablo one can opt to display an overlay map showing a semi-transparent rendering of the world that has been explored. As you explore the world, more of the map is displayed.
In this map the locations of fellow party members are each marked with a green X. In addition the name of that party member is displayed just below the associated X. All of this has the effect of creating a rather noticable pack of green when the party members travel together.
One of the skills that bardo safaris can develop is an enhanced ability to work as a group -- thus enjoying the synergistic benefit of being in a group.
When working online in Diablo, it is a simple matter of pressing the tab key to get a visual read on the POG. While it may not be as easy to *see* the POG when remoting, many report that it is just as simple. (If you are not familiar with the distinction between simple and easy, this might be a good time to acquaint yourself.)
Through working together in bardo safaris over the course of several years POG has come to mean much more than just a graphic display in a video game. And being in a POG means more than having your character in geographic proximity to other party members. There is a connection and cooperation inherent in a working POG. There is much more. But that is a good start.
In the picture above, you can see the overlay map with the tight cluster of green party members. Also visible are the same party members as they are displayed in the play view.