If you're a part of either the GB chat channel (giantbomb) or the Giant Bomb guild, you can chat with other GB'ers in-game:
1) Spectate a game
2) Hit Enter (to bring up the chat)
3) Underneath the text entry field, there should be tabs
4) Switch to the appropriate tab (GiantBomb chat channel or GB guild channel).
5) Gab on about the game while watching the game!