Euro Truck Simulator 2 and American Truck Simulator are both getting official multiplayer support, and the experimental open beta for both is available now. Multiplayer comes in the form of the Convoy mode, which lets eight players drive together in the same world: AI traffic, time and weather are all synchronized, and all eight players can be on the same job.
Both games already have mods that add multiplayer, but it's good news to have the functionality built into the game—albeit in a way that, so far, isn't quite as flexible as some community-built features.
Because it's an experimental open beta there are currently limitations. Mods aren't supported in multiplayer yet, and everyone needs to share the same DLC as the host. There are more things to be aware of, explained in detail here, but rest assured that you've got the option to revert back to a previous save game state when you exit multiplayer.
Instructions on how to enter the 1.41 Update Experimental Beta are over here. No word as yet on when the update will roll out of beta, but in the meantime SCS is after your feedback.