Google sends email notifications to all Google Groups owners to inform them about some unfortunate changes.
"Starting in January 2011, Groups will no longer allow the creation or editing of welcome messages, files and pages; the content will only be available for viewing and only existing files will be downloadable. If you would like to keep the content currently on the pages and files sections of your group, we highly encourage you to export and migrate it to another product. In February 2011, we will turn off these features, and you will no longer be able to access that content."
Google says that you can create pages using Google Sites and store files by attaching them to Google Sites pages. After creating a site, you can invite the members of your group. "Add the email address for the Google Group (for instance, firstname.lastname@example.org) with which you'd like to share the site, and select the level of access you'd like the members of the group to have."
It's difficult to understand why Google didn't automatically migrate the files and pages to Google Sites. Users could've used these features from the Google Groups interface, even if they were powered by Google Sites.
Google says that the features have been removed "to focus on improving the core functionality of Google Groups -- mailing lists and forum discussions". I don't remember seeing significant improvements from 2006, when Google Groups added the features that are now removed: custom welcome message, pages and files. Since then, Google abandoned almost all its groups and started to use the Help Forum platform. Ironically, even the Google Groups group has been shut down.