Files saved to a Dropbox Business team folder are automatically shared with your coworkers. If you don't want to share yet, add the file to your individual team member folder instead. This folder is just for your work files. And whether a file is in team folder or member folder, you can customize a shared file or folder to limit who can view and edit your stuff.
Note: While your team member folder is your space, admins can access your folder through the admin console or by signing in as your account.