In simplest form:

Code:
on *:NOTIFY:{ 
  if ($nick == Matt) { .msg $nick Hey there }
  else if ($nick == Mark) { .msg $nick Good Day }
  else if ($nick == Luke) { .msg $nick How ya doing }
  else if ($nick == John) { .msg $nick Long time no see }
}


I'd suggest however that you make use of userlevels.
Have a look at /help access levels as well as /help $level.