The Steam version provides an easy way to host as well as join other players.

There is only one internet multiplayer option which leads to a game browser.
From there you can join current sessions, or host a new one.
As a host, you can choose the mission difficulty, starting chapter, max players (2, 3 or 4), used unlockables and force hosting on a specific UDP port if needed.

One important thing to note here is that there is no cross-play between the Steam version and the InterAction studios website version, unlike in Chicken Invaders Universe.

Compared to the Steam version, the version provided by the developer on their own website requires a bit more effort to join others.

There are three different options for online multiplayer to choose from.

If you haven't done this before, you have to use the Network Wizard before using any of the online multplayer options.
It checks your framerate and internet connection performance, then presents Network Configuration settings.
Internet connection - Lets you set your internet connection type between four possible ones (Dial-up 56k, ISDN, Cable/ASDL and T1/LAN).
Max players - The max amount of players you can set your network to tolerate. With Dial-up 56k it's always set to 2, while ISDN lets you set it to either 2 or 3, and the remaining options go up to 4.
Authorization code - Required to play online multiplayer, provided with your purchase of this game.
Country - Sets your position on the world map so that you may get matched with people closer to your location for higher quality connection.

This setting will let you play with anyone playing this game on the internet.
The difficulty, unlockables and server port only apply if you're going to host a game. You can select "Ignore ongoing games" to host a new session right away. Otherwise, the game is going to search for existing sessions and try to join one.
If it fails to do that, you'll get an option to start a game.

This is similar to "Internet multiplayer", except there is no server port option, and you can enter a password. This way, you will only play with the people who know this password and have entered it as well.

This is useful in case you have other devices within the same Local Area Network. There always is a 4 player limit applied, and the first person who attempts to connect will be chosen as the host.

This section will explain how multiplayer sessions work in-game.

On the top right there is a scoreboard that cycles every few seconds, showing the Lives, Score and Ping of every player. Hosts usually have around 50 ms ping. For good connections, it usually stays around 100 ms, while problems start becoming noticeable with more than 200 ms.

You can press T to chat with other players. A single message can have up to 25 characters, and you can add line breaks with the character |. While chatting, you are not allowed to move your ship, so it may be detrimental during waves where a lot is going on.

Depending on the amount of players, the damage from your weapons gets reduced. For 2 players it's 76.92% of the normal damage, for 3 players it's 62.5% and for 4 players it's 52.63%. In Chicken Invaders 3, it was 66.66% for 2 players, 50% for 3 players and 40% for 4 players.

The manual firerate of most weapons is decreased from 6.66 per second to 5. Due to this, every weapon except for Neutron Gun and Laser Cannon cannot be overheated with manual fire.

If a player runs out of lives, they may continue playing as a "zombie", where they don't gain any points from now on and have to press the fire button to respawn. They can stay inactive for longer periods of time thanks to this, but the damage reduction will persist.

If all players run out of lives, then the session ends automatically.

Every player increases the powerup spawn limit per wave by one per player. With four players there may be 5 powerups, out of which up to 4 can be atomic powerups.

Collected keys will only be added to the player who collected them.

Cutscenes can be skipped in parts. The first player who ends the cutscene will also end it for everyone else.

Bosses targeting the ship position (Space Crabs, Iron Chef) will end up picking a random player every time. This leads to these boss fights becoming more chaotic depending on how the players are moving around.

On the InterAction studios version, the host can use in-game cheats if they are enabled for them. The cheat that adds extra lives will not work for players who have already run out of them.