Hi,

You want to allow only some users to be able to see the Design tab and add web parts? You need to grant them with sufficient permissions - by default there is a user role named Designers for this. But of course, you can modify the permissions. you can find more information in our documentation:

http://devnet.kentico.com/docs/tutorial/index.html
http://www.kentico.com/docs/devguide/authorization_permissions.htm
E-commerce guide:
http://www.kentico.com/docs/ecommerceguide/index.html

Best Regards,
Juraj Ondrus