One of the improvements mBlock made upon Scratch is the network function. You may use this feature to make multiplayer games, chat rooms, collective interactive art works or things like voting machines.
Chain mBlocks Together
In the App menu, select "Connect", "Network", it will list all the mBlock apps opening in your Local Area Network. For example, if computers in a classroom is tied to the same router, they will see each other in this menu (unless the router is configured to disable broadcasting). Click to an item to connect to a computer running mBlock (host) as a "guest".
Every mBlock App can be a "host" or a "guest". "Guest" means you are connecting to another computer and "Host" means other people connect to you. "Host" and "guests" in the same room (means guests connected to the same host and the host him/herself) can play with each other.
If you cannot see the computer you want to connect (eg. they are not under the same router), you can make a "Custom Connect" by choosing the menu item. It will pop up a dialog asking you for the host's IP Address. Google "how to find my ip address" to find the address of a computer.
One magic happens when you are connect with another computer: the "broadcast" and "receive message" blocks will apply to other users in your "room". You can learn more about these blocks in Scratch's Wiki.
If that's not enough, you can enable the "Communication" extension, which gives you a finer control over sending (via broadcast) and receiving message among connected computers.