/help on join
/help /write

Individual lines:
Code:
on *:join:[color:red]#yourchannel[/color]: {
  write nicks.txt $nick
}


Commas:
Code:
on *:join:[color:red]#yourchannel[/color]: {
  write -al1 nicks.txt $nick $+ ,
}


Just a warning with using commas on a single line... mIRC doesn't easily deal with really long lines. If this is something that you want mIRC to work with, you'll be much better off using individual lines.

Invision Support
#Invision on irc.irchighway.net