This is a lock for the L4D2 server, only people in this group can launch into the server from a lobby (but once a member starts a match, then randoms can join the server afterwards), in order to ensure the server is ready to be used by members at all times; stopping the server from being held up by random matches.
It also provides you with access to the server directly from main menu via the Steam Group Servers tab at the bottom of the screen.


If you want to play alone on the server without randoms joining, then you can direct connect to the server using the "server" alias command in your console (which you should have been given already if you were added to this group).
If a game is started via a direct connect on the server using this alias command, then only direct connections to the server will be allowed, thus randoms will not be queued in to join you. If you don't want the game to be private to must load into the server from a lobby which is described below.
If there's already a public game ongoing on the server, you may direct connect to it without problems though.


To Use The Server From A Campaign/Survival/Versus Lobby:

1.) create your own lobby (any game mode, your choice)
2.) change the lobby settings to use "best available dedicated"
3.) choose your map
4.) choose your character
5.) wait for people to join (optional, unless its a versus lobby)
6.) start the lobby and you will end up in the server. (once you're using an autoexec.cfg file with the mm_dedicated_force_servers command pointing to the server's IP)
7.) random players will then be able to join your live game in progress if you are missing any people.