Visibility can be set on your order pages to be either:
Anyone (Default) - this is your public order page
New Clients Only - any newly added client would be redirected to this page
Only Assigned Users - Specific clients added here are the only ones who has access, given that they are logged in to their accounts
To get started, go to Shopping Cart → Order Pages
Click on Manage on the Selected Order Page
Click on Settings
From there select the assignement type
Make sure to click on Save Changes