It's not that big of a deal. There's the default group, but anybody who wants to hide their files from other co-users needs to be in their own private group.
One can do a chmod to make everything unreadable by others in the group by doing a chmod -R g-rx. I'm drawing a blank on setting the sticky bit, but this help retain permissions on new files and directories under the main directory.
Or the account owner (you) can set up cronjobs that do a recursive chgrp command on a user's directory to assign all the directories and files to that private group.