There was someone working on it, not sure if they actually achieved full multiplayer. But all past attempts were a total failure as system limitations stop it from happening.
I'm fairly sure that if someone really had a ton of free time, and they had a comp sci degree, and they were an autistic savant at coding, then a split screen mod could be possible. Keep in mind, they would have to basically rewrite the graphics pipeline from the ground up (according to the devs) and tweak multiseat to work. Online multiplayer is not realistically possible due to internet bandwidth issues and computer hardware restrictions. In order to get online to work, the devs would have to compromise the physics, and simplifying the physics is not an option for various reasons. Beam has a LOT more going on under the hood than say Forza or Project Cars.
I remember the multiplayer mod by Chris Lucas (I think) it just about worked but required external software to work and as well as a goat sacrifice too it was wasnt that good to be honest, it just worked barely, so it was canned and its never spoken of again
In terms of servers (Minecraft style multiplayer- join a server, full game just with multiple people) it's not realistically possible. But Local LAN multiplayer (ie playing with someone on the same internet network as you) might be in reach, and perhaps direct connect- enter someone's IP and join thir game. That's a little more far fetched. A possible system for multiplayer would be to have one person be the "Proctor", and they're the only one who can spawn in extra cars, change map/time of day, traffic, etc. Pehaps the proctor could also choose what vehicles can be spawned, for example they can forbid the spawning of a Wentward or T-Series for lag reasons. With a basic text chat system, you've got a compelling option for roleplay, racing and offroading. I can't imagine a Beam multiplayer without chat.
Yeah it was scary stuff and it did require external software to work and it was a real buggy mess and it was chris_lucas who did it but I think he was banned a year or so ago from the forums for some reason and his account has been removed so you cant find anything on it
There has been a couple of mods, at least one of the more recent ones seems to work well. None have been released to the public though. There's also some talk of using the new steam feature with multiseat to get something working, but I have not looked into that personally. Maybe someday we will see something available and worth using, maybe we never will. For now it's multiseat or bust.