If you require more people join your games than is officially supported, (e.g., campaign with 10 players, a versus with 20) follow the steps here. The optional SteamCMD is the official dedicated server for Left 4 Dead 2.
Mouse and click over the Ip : Port of the choosen server and copy it (Ctrl + C, or right click and pick "copy") Ingame, create a lobby in your chosen game mode. REMEMBER! you have to be the lobby leader in order to pick a server. Press the ~ Key on your keyboard to open console. Paste the IP into console.
CxXV.