I think those regex octal characters do add up a couple of bytes. Perhaps they're needed in this case:

on $*:text:/^\+j\s#\S+/i:#:{
  $replace($1-,+j,join)
  .msg # $me has joined $nick $+ 's channel
}

Well, I suppose there are various ways to go about this...lol

Note: actually sunslayer's version is better. Because by using $1-, that loses the ability of stripping the control codes if entered in the trigger. Thus the code by itself won't work.