How to add some restriction on who can see the form?

I think you mean on how to restrict the form access to some people. Unfortunately, we don't have a feature that would allow you to do this within the form. A workaround I can offer would be to embed the form on your website then add the restriction there. This website should help you do the restriction: http://www.ip2location.com/blockvisitorsbycountry.aspx.