A while back I posted how I had jumped the gun on wanting the First Release option of Office 365 being restricted to certain users inside an IOffice 365 tenant (mainly to restrict the potential use of Office 2016 Preview).
The good news is as you can see from the above it has arrived in my tenant. To access it you login to Office 365 as an administrator and then select Service Settings, Updates from the admin portal. You will now see the option to be on the Standard release (slower) or First release (faster) when it comes to new Office 365 features. You’ll also see the option, if you select First release, to select those people in your organisation who will receive First release features, leaving the remainder on the standard release path.
This allows you to elect a subset of users within your Office 365 tenant who can access the new features without them becoming available to everyone immediately.