This is the central part of WP website. To secure this use password protect where the owners submit two passwords for access- one for admin area and the other for the login page.
Secure admin panel with SSL certificate implementation. This way the data transfer becomes secure between server and browser.
Multiple people access to admin panel increases security threats. Use plugins for users to make their passwords secure.
Admin as the username for administrator main account is easy-to-guess username so change this as soon as possible.
Use plugins like iThemes Security or Wordfence to monitor website file changes.