To transfer your existing save over, zip up and copy the two files of your save (including the continue file) to the following location:
/home/container/.config/Epic/FactoryGame/Saved/SaveGames
You can find your saves on your current Windows machine here:
%localappdata%\FactoryGame\Saved\SaveGames\
Once the save data is in the server location, give it a restart and when you connect to your server, you should be prompted to load up an existing save with that name.
Officially Satisfactory only supports a maximum of 4 concurrent players, however you can force it to unofficially allow more. To do this, you must change it on both the server and all clients that will be connecting.
The server side change can be found in your Startup tab under [EXPERIMENTAL] MAX PLAYERS
On each of the clients, you will need to edit your Satisfactory's Game.ini found here:%localappdata%\FactoryGame\Saved\Config\WindowsNoEditor\Game.ini
once you see the Game.ini file is open paste this line of code into a new line
[/Script/Engine.GameSession]
MaxPlayers=16