You can use the Type message key (default key - \) at any time in a Multiplayer race to type a message to the other racers. But remember, while you are typing, cars are still racing, so it might be best to wait until you are in the lobby before you make that pithy comment. During a race, use the taunt keys (default keys - 1-5) to make preset comments to the other racers.
To customize your taunts select Options from the main menu, then Multiplayer Taunts. Press the corresponding key to make that comment at any time during a race.