From CyberWurx Customer Wiki

Before you begin make sure you follow this guide: Setting your Steam Group ID

[edit] The Lobby System

The unique way Left4Dead and Left4Dead2 is set up requires a lobby system to match make and connect to games.

Sometimes, however, you want a server for you and your friends to connect to. This is where turning off matchmaking comes in.

Unfortunately, there is no way to set this in server.cfg, so the server requires you to use a file called autoexec.cfg

[edit] Creating autoexec.cfg

Autoexec.cfg may already be on your server, you can locate it in your game's cfg directory.

If your autoexec.cfg is already in place, go ahead and edit that file. If not, use a program to create the file to upload.

[edit] Turning off Lobby

Add the following line to your autoexec.cfg file:

sv_steamgroup_exclusive 1      //This will make the server exclusive to your steam group
mp_disable_autokick 1          //This will disable idle kicks if your players go idle.
sv_allow_lobby_connect_only 0

When you add the file, upload it. Restart your server to complete the changes.

[edit] Turning on Lobby

Simply comment out the settings inside autoexec.cfg so it looks like this:

//sv_steamgroup_exclusive 1      //This will make the server exclusive to your steam group
//mp_disable_autokick 1          //This will disable idle kicks if your players go idle.
//sv_allow_lobby_connect_only 0

Upload the edit, and restart your server.

[edit] Cvar Explanation

[edit] sv_steamgroup_exclusive

If set, only members of Steam group will be able to join the server when it's empty, public people will be able to join the server only if it has players.

[edit] sv_allow_lobby_connect_only

If set, players may only join this server from matchmaking lobby, may not connect directly.