either in the group forum or in the groups tab choose the search posts link. type in the person's sn and enter a date parameter. Be aware though that it can be unreliable and all of the posts may not show. Also it may not show replies the person may have made.
Another way to do it would be to type the person SN into the search field in right corner of the page. That should pull up journals and group posts by this person.
at 6:44 PM on Dec. 4, 2008