Command line utility:
Net send
Sends messages to other users, computers, or messaging names on the network.
Syntax
net send {name | * | /domain[:name] | /users} message 
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Parameters
name : Specifies the user name, computer name, or messaging name to which you want to send the message. If the information that you supply contains spaces, use quotation marks around the text (for example, "Computer Name"). Long user names might cause problems when you use them as NetBIOS names. NetBIOS names are limited to 16 characters, and the sixteenth character is reserved.
* : Sends the message to all the names in your domain or workgroup.
/domain:name : Sends the message to all the names in the computer's domain. You can specify name to send the message to all the names in the specified domain or workgroup.
/users : Sends the message to all users connected to the server.
message : Required. Specifies the text of the message.
net help command : Displays help for the specified net command.
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Remarks
?	
You can send a message only to a name that is active on the network. If you send the message to a user name, that user must be logged on and running the Messenger service to receive the message.
?	
You can broadcast a message to all of the names in your computer's domain (use * or /domain), or a different domain (/domain

omainName). Broadcast messages can contain up to 128 characters. Use discretion when you send messages to multiple users. 
Be sure teh messanger service is running on all machines.