Code:
on !*:join:#:{
  .enable #channels
  var %join.nick = $nick, %join.chan = $chan
}
raw 366:*:{
  .whois %join.nick
}
#channels off
raw 318:*:{
  .disable #channels
  haltdef
}
raw 319:*:{
  .echo -t %join.chan %join.nick is also on $remove($3-,%join.chan)
  halt
}
#channels end

Note: If a lot of people enter the room quickly, some of the people may be skipped over, due to the time required for the /names and /whois commands to be sent, and the respective raws returned.