In order to play Left4dead 2 on Garena with your friends, you will need to do following steps to ensure it works.
How to Setup Left4dead 2 on Garena
Download and apply Garry’s Patch.
Run the reg file depending on your OS (32bit, or 64bit), the reg keys should be in the reg keys folder in your Left4dead 2 directory.
Change the date to OCT 29 and time to Hong Kong (GMT+8).
Go to Garena and click settings.
Browse the executable settings file to your Revloader.exe or left4dead2.exe file. ( Run with Revloader.exe if left4dead2.exe requires steam).
Join a room and Start the game.
Note: If you are using Windows Vista then run the game in Windows XP SP2 compatibility mode as an administrator.