While it's a few weeks late, Kunos shipped version 0.9 of Assetto Corsa yesterday. This version contains the long awaited and slightly delayed multiplayer. As well as the ability to connect to multiplayer races, Assetto Corsa provides the ability to host your own dedicated server.
I'd definitely recommend running the dedicated server on a dedicated machine. Preferrably one with a good internet connection. I believe if you run the server on your own machine you will not be able to connect to it. I'm fortunate enough to have a Windows Server VPS in a data center, so that's where I'm going to run mine.
Setting up your own server is much easier than I first thought it would be. Nagivate to your installation folder (e.g. c:\steam\steamapps\common\assettocorsa) and copy acServer.exe, acServerLauncher.exe and the config folder to your server. Make a folder somewhere for it. I just used c:\Assetto Corsa.open acServerLauncher.exe.
Now open acServerLauncher.exe. This small program provides a GUI for configuring the .ini files that setup the server.
I'd only pay attention to the first tab when you're getting started. It's all pretty self explanatory. Presets are saved settings of the various configurations / race formats you want to run. To get started I thought I'd create a Practice day at Magione, the track that has been in the game since the first Tech Preview.
Select the cars you want for this session. Maybe something sensible, like all GT3 cars, but hey, whatever you want. Then in "Select to Customize Sessions, select Booking and give a booking time. The time is in minutes. If you're running a league race just supply whatever time will give people enough time to turn up at the advertised time, maybe 10 or 15 minutes. I'd keep the wait time low. Do this for each session type you want.
I would be cautious when setting Max Clients. Clients are CPU intensive and will bog down even most decentish machines. 20 is too many for my i7 2600K @ 4.2Ghz sim rig.
Now supply a server name to identify your session to your league or open world if you want. Supplying a password will make the dedicated server private, probably a very good idea!
There are other things to set but now just check the UDP and HTTP port and save the Preset. Now you're ready to nearly Launch.
Before launching you will need to make firewall exceptions on your server machine and/or perform portforwarding on your router. UDP port uses UDP and HTTP is a TCP connection so make entries for a UDP connection incoming and outgoing for port 9600 and TCP connection incoming and outgoing for 8075 - or whatever you chose for your ports.
One more step: open the server_cfg.ini file in cfg directory and ensure it has the REGISTER_TO_LOBBY=1 entry (it should - It did not in the original release, when I was trying to get a dedicated server going!)
Now you're ready to Launch. Launch and open Assetto Corsa on your games machine and refresh the server list. acServer.exe communicates with the game lobby over HTTP, getting the details for the Lobby. Players communicate with your server over UDP.
You should see something like