Use the ON JOIN event. See /help on join.
Example:
on @*:join:#channel:{
mode # +v $nick
}
This would auto-voice anyone who joins #channel, provided you're an op.
Do note though, mIRC has its own internal AutoOp and AutoVoice lists, see /help /aop and /help /avoice. These would be easier to use if you're unfamiliar with scripts.
Regards,