So the fix I found works for Linux/Steam deck users. I have no idea if it would work for Windows users but I suppose you could always give it a shot if you are a windows user. What you need to do is first navigate to the location where Steam is storing the game on your system (Typically that is going to be in your home directory in most linux distributions at the following folder structure:
/home/LOGINNAME/.local/share/Steam/steamapps/common/Grounded
If you aren't sure where the files are located you can always launch steam, go to grounded, go to options gear, then select properties, Then on the left side click on "Installed Files" at the top there should be a button that says "Browse" next to where it is telling you how much disk space it is using. Clicking that button will open up your file manager to the correct location.
Next navigate from the Grounded directory down into .../Grounded/Maine/Binaries/Win64/
In that directory you should see a couple of files named:
PartyWin.dll and PartyWin7.dll
Now you need to rename the PartyWin.dll file (or you could delete it but I prefer to rename it to something like PartyWin.dll.original)
Then rename the PartyWin7.dll file to PartyWin.dll
That's it, now you should be able to launch the game through steam and the multiplayer should work again.
Just as a note the exact error message I received when it wasn't working was:
Hosting Game Error. There was an issue hosting a multiplayer game. Check your connection and try again.
ConnectToNetwork: Failed: 3: 63
So if you are getting a different error message your mileage may vary with this being a fix. Also as I said this works on Archlinux/Steam Deck but it should fix the issue with any distribution that steam works on.
