To send a message to a logged-in user, type the monitor command
SEND NAM MESSAGE
where NAM is his programmer name (initials).  For longer messages,
just type SEND NAM and follow instructions.
If you don't know the programmer name for a user you can try his
real name.

To send a message to a particular terminal, type
SEND TTYn MESSAGE
where n is the terminal line number; to send to a particular job, type
SEND n MESSAGE
where n is the job number.

For more information, see the file MAIL.BH[UP,DOC].