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.

The following switches can be used, immediately after the SEND command name:

/M -- MAIL the message also, whether or not the recipient is logged in
/Y -- (yes) MAIL the message if the recipient is not logged in
/N -- (no) Don't MAIL the message
/W -- (where) List the job number, terminal number, etc. of all recipients
/D -- include a Distribution list in the text of the message

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