Why can I see other sites and user accounts from my multisite dashboard?

Hi, I've created a test site in my multisite network and when I log in as the new user I can see the primary site listed under "My Sites" and I can see other user accounts under Users / All Users. How do I change this so the person who signs up for a new site can only see their site and their user(s) in the dashboard view?

Kind regards
Peter