I think you just need to adjust the group settings. See:
http://groups.google.com/support/bin/answer.py?hl=en&answer=46390
The "Reply" option in a post
"The "Replies to messages" options under the "Email delivery" section
of the Group settings page refers to the reply field that is set when
a message is sent via email. For example, if you select "Replies are
sent to the author of the message," your group members who receive
messages from your group via email will be able to click "Reply" in
their email programs and the reply will be sent directly to the
author, not to the group.
When someone clicks the "Reply" link on your group's web page, the
reply will always be sent to the group. To reply to the author from
the web page, click the "More options" link near the top of the
message, then click on the "Reply to author" link."
~Rachel