The system settings can be found in the dropdown menu in the right top. Only users with the Admin role can see and access this menu item.
In the first instance the "General" tab will be opened. If you want to change the system title and icon, you can do so using the "Name" and "Icon" fields. Everyone who uses the system will see the new branding.
By default, anyone who types in the url where you installed the system can sign up. No need to worry, guests will get the Client role by default and can not do anything other than customize their profile. But you may want to disable registration. If you deselect "Visitors can sign up", there will be no registration link and page, and you can only add people with the user management.