As of the server initialization (Nov 22, 2021), to join multiplayer servers, your game client needs to be the experimental build.
Private Community Server
Status: onDemand (inactive gets shutdown but saved) Requested Launching via Discord
Server configuration and settings are all default
None currently installed.
Experimental how to:
If you got Satisfactory via the Epic Games Store, you can simply download and play the version of the game labelled "Satisfactory Experimental" which you'll find in your games library.
If you have the game of Steam, you'll need to right-click on Satisfactory in your games library, then navigate to Properties > Betas, and select "Experimental" from the drop-down menu under "Select the beta you would like to opt into." Steam will then download the experimental version of the game.
It is advised that before doing this, make sure your save files are backed up just in case something goes pear-shaped. Your save files should be located here: