The easiest way to find your GUID while in game is to type a / as if you are going to direct chat in game and then type in #beclient guid. After hitting enter you will see battleye display your GUID in the lower left corner of the screen. Now grab your pen and jot down the GUID battleye displays.
You may need to repeat the process because of the length of the GUID and the speed that it fades from view.
See more information at: