You can now gather your friends, and test out the Project Zomboid multiplayer. It isn't an official release, so you have to follow instructions to activate Steam multiplayer on the Indie Stone forums.

Very much like the main game, the multiplayer is still in the early stages of development, thus has a lot of bugs. The developers have warned about multiplayer not being ready for "the main branch". The most common problem is difficulty joining other's servers. The last update fix was released last weekend. Unfortunately, mods are currently not supported in multiplayer.

Project Zomboid seems to be a game that was meant to be multiplayer, but I think I'll wait around for the official beta release to come along. I played the early access game around November, and I remember the incredible frustration I experienced due to being stuck in various inanimate objects, which forced me to restart the game. I can't imagine how the same game would be, with multiple people stuck in the counter, while zombies force their way into your hideout. I'm aware that many bugs have been fixed since, but I personally would rather play a more stable game. This information about multiplayer has re-sparked my interest though.