It seems I'm not the only one who finds the removal of the google groups advanced search extremely annoying. Threads such as here abound...
The support page is broken and useless because Google's been rolling out interfaces faster than they can write documentation: http://support.google.com/groups/bin/answer.py?hl=en&answer=46036
Googling for the groups advanced search page turns up a lot of broken search pages (for example: http://groups.google.com/a/webmproject.org/) which have the search form, but yield no results of any kind for any searches.
So, in hopes of helping anyone else out who is looking for this feature, I found this working link buried in the cached version of one of the google groups pages:
When Google took over Deja many years back, they took on the responsibility of maintaining an archive of internet historical significance. It may not seem like much to some, but I love that you can go back and see the first emoticon discussion, or see some of the earliest successful open source, or early posts and discussions from true icons of technology.
Who knows how long that link will keep working before some UX "engineer" will see fit to burn it down completely. After all, if we're too stupid to handle advanced searching, we're too stupid to care about some old boring old "forum" posts.