At present it is not possible to join a multiplayer event. We will fix this in tonight's update. Once it is possible you do it as follows:
The in game server list does not presently list servers.
Firstly make sure that Automobilista is not running.
To view servers with in your steam client click 'View - Servers' Then under filters, choose Automobilista for Game.
We will in future add a better method for joining online servers.
Thank you.