Default L4D2 Config not allowing lobby
From CyberWurx Customer Wiki
New installs after November 20, 2009 can disregard this message
The default server.cfg had the following CVAR enabled:
sv_steamgroup_exclusive 1
This is possibly restricting lobby players from entering your server. If this cvar is set in your server.cfg and wish for the servers to be filled, please either comment this cvar out, or set it to 0.
//sv_steamgroup_exclusive 1
OR
sv_steamgroup_exclusive 0
For more information about sv_steamgroup_exclusive, see Setting your Steam Group ID and Turning off the Lobby System