How to send multiple computers a message with the msg command?

Question

I want to send multiple computers a message with the msg command. This command is specific to windows/DOS.

I know these instructions:

Send a message to a user.

MSG {username | sessionname | sessionid | @filename | *} [/SERVER:servername] [/TIME:seconds] [/V] [/W] [message]

username            Identifies the specified username.
sessionname         The name of the session.
sessionid           The ID of the session.
@filename           Identifies a file containing a list of usernames,
                    sessionnames, and sessionids to send the message to.
*                   Send message to all sessions on specified server.
/SERVER:servername  server to contact (default is current).
/TIME:seconds       Time delay to wait for receiver to acknowledge msg.
/V                  Display information about actions being performed.
/W                  Wait for response from user, useful with /V.
message             Message to send.  If none specified, prompts for it
                    or reads from stdin.

My question is: What is the structure of @filename and how to build it?

My network is Workgroup.