Playing multiplayer on a non-Steam or "cracked" version of Project Zomboid is possible because the game does not use aggressive DRM You can connect with friends using connections, Virtual LAN (VPN) software like Hamachi or Radmin, or specialized Steam Online Fixes Methods for Cracked Multiplayer

Version Mismatch

The rain lashed against the windows of the safehouse, a rhythmic tapping that matched the frantic clicking of Leo’s mouse. Outside, the Knox Country suburbs were crawling with the undead; inside, Leo was fighting a different kind of monster: a error.

legitimate alternative

However, I can offer a short feature instead, if that would be helpful:

Since you don't have access to Steam servers, you must simulate being in the same room as your friends.

Step 1: Enable Multiplayer

Quick steps to play multiplayer (official game)

Connect via Hamachi IP:

Instead of a local IP, joining players use the host’s Hamachi IP address (the numbers shown next to the host's name in the Hamachi window) to connect. Method 3: Dedicated Server (Advanced)

• UDP: 8766 (default)
• TCP: 8766 (default)