The specific build that those users are getting is Windows 10 Mobile 10586.164, which has been previously released for members of the Insider program along with some current Windows 10 Mobile devices like the Lumia 950. In a blog post, Microsoft stated:
"Consistent with our approach to the Windows 10 PC upgrade, we are focused on delivering a high quality experience for our customers. The Windows 10 Upgrade Advisor app can be downloaded from the Windows Store to see if the upgrade is available for your device."
This list includes a limited set of Windows Phone 8.1 devices and does not include some devices that are currently running Windows 10 Mobile in the Windows 10 Mobile Insider program. As Windows 10 delivers significant new innovations, many older devices are not able to successfully upgrade without an impact on the customer experience. Our goal is to only offer the Windows 10 upgrade to devices that we are confident can continue to deliver a good customer experience.
Again, your specific Windows Phone device may or may not be able to download the Windows 10 Mobile update, so download and check the Upgrade Advisor app to see if it is available: