The new ‘Select People’ option is being rolled out over the next few weeks, according to a blog post from Microsoft announcing the changes. “With this newly added option, you can select a set of users to participate in First Release,” the post notes. “These users will receive updates before the rest of your organization, giving you time to prepare for changes.”
First Release currently works for Exchange Online, the Office 365 navigation bar, and the Office 365 Admin Center. Microsoft says it will expand the range of services included in the program in the future.
Manage change and stay informed in Office 365 [Office 365 Blog]