You can open the friend window by clicking its icon or pressing the B key.
The window is composed of 3 lists: Friend, Blocked and Guild.

Lists on the Friend Window

Friend List You can add characters to your friend list and manage them by groups. Use the [Add Friend] and [Add Group] options at the bottom of the friend window to edit your friend list.

Friend List Options

Friends
Displays the names of your friends as well as their login status: online or offline.
Press the + key to check your entire friend list, and press the - key to just check the friend groups.

Whisper/Edit Memo/Delete Friend/Invite Party
Choose and right click on a friend name to use the commands.

Blocked List Options

Add Character
Enables you to add a character to your blocked list.

Release
Enables you to release a character from the blocked status.