There are a couple of ways. If your network is configured to soft delete, then the data is there forever in your exports, even if you delete the group.
However, I think a much more useful way is to change a group's status to private, set it to not be listed in the directory, and then remove all the users from the group, except a small group of administrators. The admins can get to it normally and can add people at any time. The content won't appear in searches (which are security-trimmed), nobody will be able to join the group, and nobody will even know that the group still exists, unless they try to create another group by the same name. If you want to recycle the name, simply rename the archived group (add "-Archive").