first you probably need to login to the machine with ssh.
then use write to start a message
see these
man write
man wall
man mesg
man who
it goes like this..
login with ssh. then...
use who to see where they are logged in..
who
david pts/2 Jul 4 23:30 (dallas-209-210-115-237.safepages.com)
use write to start message..
write david pts/2
Hey!
Wassup?
use ctrl_d to end message
what the other end sees..
Message from david@
www.dcphillips.net on pts/3 at 00:05 ...
Hey!
Wassup?
EOF
they can do the same to write back