Option 1: Click ‘+’ sign on bottom right corner of the main menu -> Friend -> the magnifying glass’ -> Type the in-game name that you are searching for. In-game names are case-sensitive.
Option 2: If their name shows in the World Chat, click her/his profile to add her/him to your friends list.
If you and your friend don’t share a server in the same time zone, you can’t search for your friend.