First, sorry for my bad english... ^^ I am from Brazil.
Well, my sugestion is we , have a chat in Game...
Exemple:
In game we gave a window with the name VIP list, there we can put name of friends... When a person enter in game... his/her name will turn green when enter in the game, and when his/her name turn red, is becouse he logout of the game.
Ok, and what now i can do with that?
Well, i dont see at game a command for use when i want talk in particular with another player...
My second sugestion is, when you click in his/her name in vip list, open a small window where you can talk to the person...

I dont know, if another user post somthing sugestion talking about that, but its a sugestion... xP
Bye ^^