Keep track of your friends and foes with the Friend and Gag list.

The lists are accessed in the right side of the lobby window.

At the bottom of a players profile and at the friend list you can add your personal notes on a player.

If you add a person as a friend, you will be notified when that user logs on with a message and a sound.
(sound and messages can be customized in Settings)
If you select gagged you will no longer be able to hear this persons messages in public or private chat.