Windows Insider Program allows you to test unreleased Windows 10 versions. It is a formal process, and the option to register for Insider program is baked into the system. That said if you are unable to register because the Get started button greyed out, then here is what you need to do to enable it.
Get started button greyed out – Windows Insider Builds
The Get Started button is usually greyed out when the Telemetry is not enabled. Telemetry or Diagnostic feature allows Microsoft to collect data from Windows, i.e., diagnostic data.
Enable Full Diagnostic Data
Enable Telemetry data via a registry
Use Group Policy to enable Telemetry
Windows users need to enable this feature in full to enable the Get started button.
1] Enable Full Diagnostic Data
To enable Full telemetry, go to Settings > Privacy > Diagnostics & feedback> Diagnostic data. Choose Full as the option.
It is vital for insiders for its complete evaluation of the current system, and help Microsoft to understand the error, and resolve them before the final release.
2] Enable Telemetry Data via Registry
If you like editing via the registry, then it’s easy as well. It is useful when you want to remotely enable this feature on a computer connected on a local network. You can even export the registry key, and then install it on multiple computers.
Anand Khanse is the Admin of TheWindowsClub.com, a 10-year Microsoft MVP Awardee in Windows (2006-16) & a Windows Insider MVP since then. Please read the entire post & the comments first, create a System Restore Point before making any changes to your system & be careful about any 3rd-party offers while installing freeware.