To start a multiplayer game, you and your friends will need to be connected to the same network. All mobile devices must be on the same network and multiplayer version in order to connect to one another!
When searching for a game, the Multiplayer Version Number can be found on the bottom-right corner of the Join screen.
Players may also experience connectivity issues due to router and firewall settings, which would need to be configured properly according to the specifications and documentation included with your router and operating system.