(v2020.12.19i_multitest):
You can access this version by right clicking the game in you steam library, selecting properties, betas tab, and from the dropdown select "multiplayer_test".
I don't know where to start, so here are a few images showing how much fun it was to get to the current state and a video, which shows what's up.

(meatbag)


(flying meatbag)

(mustache)

(almost...)

(there!)


(uplifting result)

(there is someone on the rear seat!)

(the rotation is still not good sometimes)

(leaning out of the window)


(parts fell down for both players)

(crouching is... fine...)
The following things are synced now:
So there are still many things missing. Like the content of fluid containers, engine and car state, light switches, shooting eachother...
And there can be errors sometimes, like one of the players freezing for a few seconds and then until they open the menu, they have the default outfit.
New items spawning, which are not yet there at the start will be most likely different kind of items for each player.
Terrain may not be the same near the starting house.
I'm not sure what's up with the thing I said last time about the floating origin problem, it seemed to be working now, but I won't believe it yet, seems too good too be true.
Compared to the previous version, it almost looks playable now, I hope I will be able to say something like this again soon! (that the new version will be better, as much as this is compared to the last one) ːsteamhappyː
And we have a new discord server, because the old one was not really ours, we will have more control and hopefully things will be more in order.
The old one will work until around christmas.
https://discord.gg/ftxZDVEzzT