Click on the icon placed next to the game-money display to activate Messenger Function.
Messenger Function allows you to Add, Remove, Block friends, Group management, 1:1 conversation with your friend and etc. Some of the functions can be used by typing in orders in the chatting window.

Friend related Chatting Orders

/block [character's name] : Block specific friend's whisper
/add [character's name] : Add to the friend list
/remove [character's name] : Remove from the friend list
/cancel block [character's name] : Undo whisper block

Messenger related chatting Orders

/add group [group name] : Add group to the messenger
/remove group [group name] : Remove group from the messenger
/move group [character's name][group name] : Move the character to the group