You can randomize them this way:

on !*:join:#channel:{
  tokenize 124 $+(msg #,|,notice $nick,|,msg $nick)
  $(,$+($,$r(1,$0))) Entry Message
}

If you already specify the targeted channel in the join event, you don't need to respecify it as "msg #channel," just "msg #" or "msg $chan" will get the job done. The exclamation prefix by the join event is to tell mIRC not to trigger and send the greetings upon the client that runs the code. Think of it as if $nick != $me. Unless, of course, you like to send a welcome message to yourself whenever you join a channel.

P.S. I know this is an old snippet from last year, but I thought I'd offer my input.