This is basically the same question you asked here:
http://www.yabbse.org/community/index.php?board=141;action=display;threadid=23431. And you received an answer from [Unknown] that you can't do that without you, or someone else modding some files. He even said he might do it. So you could just stick it out a bit, or if you know php/mysql, try to do it yourself. Aquilo, in that same post, told you what it basically involved here:
QuoteA mod would have to be made in Post.php to make an sql query find which members are in your group, make an array of there name's/email address's and loop the mail() function when some one posts a new message.
Methonis