You can now join the L4D2 Server ICED is running just be being in this group.

You do not need mods nor do you need to be using the same mods as everyone else but this collection can get you started with a base https://steamproxy.com/sharedfiles/filedetails/?id=2501477450

Will be doing the Back to School add-on campaign Thursday so have that downloaded at least (all 7 parts plus addendum)