Can I restrict new team member's access to a single folder?

Hi Terry White !

You can absolutely do that.

You first need to change the access for that Team Folder, to remove the "Everyone at [your company]" group. To do so, navigate to the admin console, and click on "Content" in the left nav (https://www.dropbox.com/team/admin/content/home). Then click the "..." on the right side of the folder you're trying to restrict, click "Share with Dropbox", then click the "Can edit" button to the right of "Everyone at [your company]" and select remove. You'll need to add back the people who do need access though. 

Let me know if you have any additional questions!