Here's the basic idea including "error" messages.

Code:
on *:text:!msg *:#: {
  ; Check to see if the nick is in the channel.  If not, display a message and halt (return).
  if ($2 !ison $chan) { msg $nick That person isn't in the channel. | return }
  ; Check to see if there was a message included.  If not, display and error message and halt (return).
  if (!$3) { msg $nick Please include a message to send to $2 $+ . | return }
  ; Send the message ($3-) to the nick ($2).
  msg $2 $3-
}

Invision Support
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