...and if you're wishing to /whois everyone who joins a specific channel, then you will need to use the ON JOIN event, see /help ON JOIN.
Example:
on *:join:#channel:{ whois $nick }
With a large channel this could cause you to lag. Obviously, replace #channel with the channel you wish to perform the /whois on, or # if you want to perform it on all channels (not recommended).
Regards,