I've seen a couple of comments saying you can't registeras an Insider, but I just wanted to say I've done it today and it still works, just in case it helps someone.
What you can't do (well not officially; I imagine there are workarounds) is activatea new build following the temporary change announced here:In my case, I had a VM installed ages ago which only had a local account, and I wanted to add a different MSA (which didn't have Insider status before today) so it would pick up the upgrade to 10240.
The Insider webpage let me register and join the Insider programme and the Notifications saying I need to do things to keep getting Insider builds have gone away, and it can now 'see' 10240.
I think things changed in the last few hours. Now only mobiles can be integrated in the Insider Program.
Home page - Windows Insider ProgramNOTE: The public release of Windows 10 is near. So we are not integrating new computers in Windows Insider Program. You can still register and begin using Windows 10 Mobile Insider Preview. We will inform you as soon as there is a new preview version of Insider for you to try on your computer.
That's what the page said when I registered.
Although you can't activate a new computer, you can still register a new email address as an Insider (for one thing the Mobile builds are still available as you say).
This then means I could add an MSA as an Insider to a computer where it hadn't got one previously. This worked and I've got rid of the Fix Me message.