You can limit who can send links, invite collaborators, and or turn off sharing in order to limit access to your files and folders. You can set access limits on every folder and file you create by default via the "Content & Sharing" tab in your Account Settings.
To set access permissions on a folder-by-folder basis, select the to the right of your folder name, then select the"Security" option in your Folder's General Properties menu. Full details about how to set different permissions can be found in the Smart Links support document on the Box Support Site.
Note: You can invite collaborators to your folder at the time of creation, or you can change a private folder to a collaboration folder at any time after the folder has been created.