When you join a channel mIRC knows of only one user, you. You have to wait till the names list has been recieved, which is Raw 366 - End of /NAMES list.
Code:
on me:*:join:#:{
  inc -u99 %joined. $+ #
}
raw 366:*:{
  if %joined. [ $+ [ $2 ] ] {
    var %i = 1
    while  $nick($2,%i,ov) { echo -a $ifmatch | inc %i }
    unset %joined. $+ $2
  }
}