Yeah, the PC users need to figure this out on your own.
The console versions have built in chat, as the consoles in general already have built in chat. I played the campaign with my brother pretty much all the way from start to finish with him, chatting the whole time over X-box Live.
I really think this is more of a console game anyway. I've had a lot of problems with PC games that also have console versions of them. Some won't run (dragon age origins, Fallout 3), others have goofy controls you can't really customize (Battlefield Bad Company 2).
I've got a rule now that if I can get it on the console, I will. I even picked up a used copy of Battlefield Bad Company 2 for my X-box 360 this week, even though I have the PC version. I found the controls to be a little slower, but that also meant that the other players were likewise slower. It actually made the game feel better as I had some time to see people before the shooting takes place. On PC you get shot a lot easier, and snipers are a lot more common. Bad Company 2 is still a twitch game, but not as insane on the X-box.