Steam Workshop makes it easy to add mods and some maps. Here's a short list of mods definately worth getting, and none of them confict with each other.

Custom Crosshair White Dot (medium-big/outlined) BETA
Urik Color Icons
CrazyRabbit's Tactical Flashlight
Rocker Rochelle
New Weapon Sounds v1.2b
Nick's Black Suit
Improved Blood Textures

What this means is you can now ignore the previous post and just use the server by choosing steam group as the server type, like you could before the Christmas update.

Let's hope they now get around to fixing the lobby chat bug soon.

OK, so you've followed the previous instructions, go to look for the "Cheesy Custom L4D2 Map Server" and dont find a listing for it. WTH is going on?
Well the simple answer is, what GabeN gives, GabeN can taketh away. That same "update" that broke the talking in lobbies, also broke the steam groups feature. Hopefully Valve will get around to fixing this in the new year.
There IS a way to get on the server, its a little complicated. In your folder located at: "..\Steam\steamapps\common\left 4 dead 2\left4dead2\cfg" you need to have a file named "autoexec.cfg". If you don't have it you can make it with Notepad. In this file you need to put the command: "mm_dedicated_force_servers 8.6.74.196:27015".
If there is a reason that you have this file already, you can put the command anywhere in it on its own line. I recommend at the end. Save and restart L4D2, and you are good to go.

WARNING: DOING THIS FIX WILL ALWAYS MAKE YOUR COMPUTER TRY TO USE THE SERVER IF YOU ARE THE LOBBY LEADER NO MATTER WHAT SETTING YOU PICK, EVEN "OFFICIAL DEDICATED". IF THE SERVER IS BUSY WITH OTHER PLAYERS OR DOWN, THEN IT WILL GO SOMEWHERE ELSE.

Any member is able to play on the server whenever they wish as long as there are open spots on the game.
If you want to play when no one else is around, make your own lobby and choose "Steam Group Server" for server type.
When you start the game, a list of all the servers that you can play on for ALL your other group memberships will appear.
Ours is called "Cheesy Custom L4D2 Map Server". Select that one and go.
Happy Zombie Hunting!

PS: For those of you to whom it might mean something, the server's address is 8.6.74.196:27015

While the group does have an "invite only" membership, anyone who is a member can invite others to join. There is only one requirement: the person has to have a team-oriented play style.
If you know anyone who isn't a member currently and fits the bill, please feel free to invite them to join.