Here's the full text of the Win2K help page on the command "net send":
net send {name | * | /domain[:name] | /users} message
Parameters
name
Specifies the user name, computer name, or messaging name to send the message to. If the name is a computer name that contains blank characters, enclose the alias in quotation marks (" "). Long user names may lead to problems when used as NetBIOS names. NetBIOS names are limited to 16 characters, and Windows 2000 reserves the 16th character.
*
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. If name is specified, the message is sent to all the names in the specified domain or workgroup.
/users
Sends the message to all users connected to the server.
message
Specifies text to be sent as a message.
Hope this helps!