I am providing the full guide how do u send message…..
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.Top of pageTop of page
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:DomainName). Broadcast messages can contain up to 128 characters. Use discretion when you send messages to multiple users.
/users lets you send a message to all users who have sessions with the server. Use discretion when you send messages to multiple users.
The Messenger service must be running for messages to be received. For more information about how to start a service, see Related Topics.Top of pageTop of page
To send the message “Meeting changed to 3 P.M. Same place.” to the user robertf, type:
net send robertf Meeting changed to 3 P.M. Same place.
To send a message to all users connected to the server, type:
net send /users This server will shut down in 5 minutes.
To send a message that includes a slash mark (/), type:
net send robertf “Format your disk with FORMAT /4″
Note: Remember u must start the messenger service from Windows Service box