You'd want to /whois the nick that joins the channel and then respond to the channels returned.
What you want to respond to is a raw numeric. To find out what numeric it is, you can use mIRC debug function, e.g. /debug @debug

Your script would then start like this.
Code:
raw 123:*:{ response code here }

123 is the example raw numeric here. The real numeric is *probably* 319