An "insider" is someone who has special knowledge or information about a particular group, organization, or situation because they are a part of it or have access to important details. Full definition
Get the best of business insider delivered to your inbox every day. (fsat.ru)
It's a part of what you've signed for as a beta (windows insider) program user! (tomshardware.com)
It's also worth noting that users who are in the Skip Ahead ring don't need to join the app insider program because they're already eligible for the app updates. (windowslatest.com)