One player must Start the game with Multiplayer (host game) checked.
All other players must Join the game. Wait for the server to show in the online servers list, select, choose your User / Nickname and Password so you can identify yourself in the server.
Address: IP address of the server
User / Nickname: Your nickname
Password: Your password
The online servers will only show other devices in the same local network. If you want to play via Internet, the host will have to open the port 9090 (NAT) and provide the external IP address to the other players.
The multiplayer mode only works via Wi-Fi, and is limited to 2 players in the free version.