"Over the next few months, Facebook will be archiving all groups created using the old groups format. Moving forward, you can create groups using the new groups format, which makes it easy to share with the important groups in your life."
As part of the archiving process, members of the old group get deleted. In other words, "archived" groups are destroyed.
While facebook claims that it is possible to upgrade to the new groups, this option is not available for larger groups, which are most at need of it.