Code:
;on text if someone says !yell in a channel (#)
on *:text:*!yell:#:{

;count the number of nicks in channel and store number to a local variable %c_
  var %c_ = $nick(#,0)

;while %c_ is not $null
  while (%c_) {

;message in channel that users nick.
    msg # $nick(#,%c_)
    dec %c_
  }
}



This will respond to anyone saying !yell. You can use mirc user levels or write code for it to only respond to specific nicknames. I didn't want to make it more complicated. Let us know if you need help with this.