To add your server, you need this Steam group's ID.
(When you join this group as a player, you will see all servers that use this group ID.)

If you own a multiple slot server, you will need to add this group's ID to the "sv_steamgroups" command within your server's server.cfg file.

Example:

sv_steamgroup 0011223344

The group ID is not 0011223344, but private to avoid spam. To get it, send an email titled "Multiple Slots L4D2 Servers" to yvoman+msl4d2@gmail.com with your server-IP, servername, total slots and country.

You will get the group's ID emailed back to you asap.
You can also try to contact other multislot server owners and ask them for the ID. Do feel free to post your server details in the group comments for others to see.

Thanks and have fun.