All posts to the discussion groups are archived for future reference. To access the archives, you must have a Google account subscribed to the group whose archives you wish to search. If you do not have a Google account, you can create one for free here, then send an email to email@example.com (using your email account that is currently subscribed to the group) and ask to have your new account added to the group.
Once your Google account is subscribed, simply go to Google Groups, click “My groups” and then click the name of the group you wish to search. Using the search bar at the top of the page, you can search the archives for your topic of choice.