UPDATE: You guys no longer need this because the fix has been officially released for the Alien Swarm SDK with the Jul 6, 2012 update.
I have decided to release the matchmaking.dll files which allows your alien swarm mod to have more than 4 players. By default your ASW SDK mod will be restricted to maximum of 4 players due to some code in the matchmaking.dll which nobody has the sources to. So some time around
July 2011 we've contacted Valve's Jonathan Sutton ( creator of asw ) and he was nice enough to send us modified matchmaking binaries. The main reason I've decided to release this is because of my astonishment that Valve has not released this yet ( Its almost been a year ) as an update to the Alien Swarm SDK.
Here are also some instructions from his email.
Jonathan wrote: Okay success! I found another spot that was limiting the slots. This should work now. Can you please make sure the line below is in your asw_gamerules.cpp, recompile server+client dlls, then use the attached matchmaking dlls. Let me know if it works.
ConVar mm_max_players( "mm_max_players", "32", FCVAR_REPLICATED | FCVAR_CHEAT, "Max players for matchmaking system. Mods that support more players can increase the default value here." );
One thing to remember:
The number of slots cannot be changed once the matchmaking lobby has been created. The limit gets set at creation time. So you must make sure the convar mm_max_players has the desired value before the lobby is created (i.e. before g_pMatchFramework->CreateSession is called). If you only have one game mode, you could just set the default in your code to 32 like is shown above. When I ship this as an update to public, it’ll be set to 4 there.
You may also want to to look at GetPlayerLimits in asw_gameinterface.cpp if you haven’t already. That sets the bounds for maxplayers in the engine.