Multiplayer allows playing with other people together at the same time. It is possible to play Project Zomboid either locally (or via Steam remote play) using splitscreen or online around the world with online multiplayer.
Splitscreen was first added(When?) in a rather buggy state, and was being worked on to be fixed. It was later re-included in a future build(Which one?). Splitscreen multiplayer allows up to 4-people playing together with just one copy of the game, either locally on the same PC, or via remote play, though only one person can play with keyboard and mouse. Splitscreen multiplayer isn't as demanding as online multiplayer and for a while it was the only option to play together in initial build 41 versions.
Online multiplayer was released to the public as a beta on 10th of February 2014. It was available until build 41 when it was temporarily disabled and tested internally, until its reimplementation in final Build 41 releases. At this point in time, the multiplayer, much like the rest of the game, is still being worked on with improvements already in queue for Build 42. This means that bugs and issues will usually come up during the course of play of the game. If you encounter issues while playing the multiplayer, you can try and see help on the steam forums or can visit the "Multiplayer Help" section of The Indie Stone forums.
In the main menu you will find a "JOIN" and "HOST" options. Clicking on the JOIN brings the connection screen. You'll need a server to connect to – you can either use the server browser, find a list of servers in the "Servers" section of The Indie Stone forums or connect to any server you have address of.
Some servers will require whitelists. This means that the owner will need to add you to the database in order to allow you to connect. If you run into general multiplayer problems you should post in the Multiplayer help section or the Steam forums.
Below are some helpful links that should help cover subjects like FAQ, server set-ups etc.
- Multiplayer FAQ
- Server settings
- Startup parameters
- Dedicated server
- Buying and running a Linux Project Zomboid server