It's called "Versus" mode and it's 4vs4 max. 4 survivors and 4 players who can control zombies - you can choose between boomer, hunter, smoker and tank... and basically do the job instead of the AI.
BTW there is a traditional server browser which apparently currently doesn't allow you to join empty servers. It was quick-fixed by Valve after people complained that the process of joining a server (random) is annoying. Hopefully they'll expand it to a proper menu option by release or later.
For now, to access it:
"Simply enable developer console by going to Options, then Keyboard/Mouse, then 'Enable Developer Console'.
After that, hit the ~ key to open up the console and type in "openserverbrowser" (no quotes). This opens up the server browser you all know and love. Right now, all games are listed under the Custom tab."
- left4dead411.com
Here's a vid of the Versus mode too. It used to be available via an exploit in the demo (!) but Valve patched it so it no longer works.
[yt=480,385]H2doGzoRf2Q[/yt]