if they don't have any servers to host, it should be like minecraft, you make your server and host it with your computer
As far as I know, there is no official one, but something else is in development, I believe it was called BeamMP.
The developers say that they plan on adding it on the future, but at the moment they are focusing on other aspects of the game. I'd also love to see it but I guess it is by no means an easy task, and I doubt they'll have time to implement it any time soon.
This has been discussed many times before (someone makes a new thread maybe twice a month...). You can search for an existing thread on the subject for more details, preferably before making a new thread for it. Users and staff alike have gone into great detail about the issues previously in those threads. But as a recap: Local splitscreen is a fair challenge, network multiplayer is as well. Currently it is more of a research catagory opposed to anything users can expect. We do have multiseat, and there are programs you can use to stream that. Depending on what you're using it for, that can work fine for network multiplayer. A few modders have tried their hand at this, and currently the one already mentioned by another use 2 posts above is in a very good state and perfectly reasonable for just having fun with a friend.
Oh good, another thread about multiplayer. Seriously? The devs don't even have any plans to add it right now. Look, I'm aware there's a WIP thread for this feature, but there's been enough threads about this subject in my opinion. Look, I'm not saying multiplayer is bad. It might be kinda cool to have it in BeamNG, but it's a feature that can't be added to the game right now. I'm just really fed up with pointless threads about this, tbh.