[cammie:~] pergesu% man wall
WALL(1) System General Commands Manual WALL(1)
wall - write a message to users
wall [-g group] [file]
Wall displays the contents of file or, by default, its standard input, on
the terminals of all currently logged in users.
Only the super-user can write on the terminals of users who have chosen
to deny messages or are using a program which automatically denies mes-
-g Send messages to users in this group. This option may be speci-
fied multiple times, and any user in any of the specified groups
will receive the message.
mesg(1), talk(1), write(1), shutdown(8)
A wall command appeared in Version 7 AT&T UNIX.
4th Berkeley Distribution June 6, 1993 4th Berkeley Distribution
I didn't get it to work though. Another option is writing a simple perl or shell script, that does "who", gets and parses the output, and iterates through the users and sends them messages. I don't know Perl, so I can't write it. But I'm sure it wouldn't be hard at all.