You need Freejoin to get on the practice server (this way anyone with the password can join, reserves included)
Here it is:http://www.kegetys.fi/freejoin/
Installation is as simple as it gets, unpack the files in your steam/steamApps/common/AssettoCorsa folder
You can backup some file there (in readme), but it's not really necessary; you can always ask steam to validate your installation, and that'll replace the file(s) that were changed.
You'll probably have to re-install this app after upgrades to AC.