Very simple join message/counter.

Type /on #Channel to turn the message on for a channel.
Type /off #Channel to turn the message off for a channel.

If you want to edit the greet message, feel free.

alias on {
  if ($1 == $null) { echo -a Invalid Syntax! /on [CHANNEL] | halt }
  if ($left($1,1) == #) {
    set %grt [ $+ [ $1 ] ] on
    echo -a Greet message for $1 is now on!
  }
}
alias off {
  if ($1 == $null) { echo -a Invalid Syntax! /off [CHANNEL] | halt }
  if (%grt [ $+ [ $1 ] ] == on) { unset %grt [ $+ [ $1 ] ] }
  else { echo -a Greet message for $1 is already off!
  }
}
on *:JOIN:#: {
    if (%grt [ $+ [ $chan ] ] == on) { inc %grtj [ $+ [ $chan ] ] | notice $nick Hello and welcome to $chan! I've seen %grtj [ $+ [ $chan ] ] people join here! }
  }
}