They recommend a 2nd install of GTL for altbierbuden, I think they also recommend the non-steam version but, honestly, I'm not sure it matters. I use a 2nd install for it as I use my regular GTL install for my career mode stuff with mods like the GTL HQ & Anniversary mods. Then I have another install just for multiplayer / Altbierbuden.
To set it all up, IGNORE the first 2 steps of the instructions on the altbierbude website and in their readme file where it says to download & install Python and wxPython. Just download & install the autoupdater as that will also take care of Python & wxPython for you. It'll also install the correct versions as the manual Python & wxPython are slightly outdated versions. I recommend installing Python & wxPython in the default location (C:\Python or whatever).
During the install process, autoupdater will ask you to create a separate folder for all the downloads to save. I personally recommend setting this to a backup folder / drive. There is around 25 GB of cars, tracks, etc. to download. If you ever uninstall the game, upgrade to a new PC, etc. you can just point the autoupdater to this folder and it will scan and retrieve all the content for the new GTL install without having to re-download everything. It will then only download any new content or any content that requires updating.
The autoupdater has a download cap of around 300 kb/s so just let it run for around 1 day and it should finish downloading and installing everything. If you donate some money to them (donate link in the autoupdater program) then it will finish much, much quicker as you will have unlimited download speed for a certain amount of time. I cant remember how long; a few months? 1 year?
LowGuard - their anti-cheat program - isn't required but recommended. It's a separate download & install (ie. not part of autoupdater).