The syntax of this command is:
NET SEND
{name | * | /DOMAIN[:name] | /USERS} message
Yes there is an option for a domain or users. I tried sending it to the Administrators group and I got this error:
C:\\>net send Administrators Test
An error occurred while sending a message to ADMINISTRATORS.
The message alias could not be found on the network.
More help is available by typing NET HELPMSG 2273.
I think it expects a systemID for the name field.
I remember this before, I had to send a message to only certain users and basically had to create a VB program to read a text box for the message and another one for a list of users' workstation names to send it to. It would loop through each stationID and shell to DOS to send the message one at a time.
Sadly I left the code for that behind at the workplace I was let go from. It should not be that hard to recreate, but alas I don't have the time to recreate it.