The First Release capability allows organizations to be prioritized in receiving updates to the service. You will receive updates more quickly. This makes sense for some organizations but sometimes large enterprise organizations with complex IT environments will not want to do it. A middle ground was introduced several months ago so that “select people” could receive the First Release features which is very helpful for large organizations with complex IT environments.
There have been some recent updates. SharePoint Online, OneDrive for Business and Office 365 ProPlus have been added to the First Release for Select People. Exchange Online is already available.
ResourcesOffice 365 First Release - https://support.office.com/en-us/article/Office-365-release-options-3B3ADFA4-1777-4FF0-B606-FB8732101F47
Expansion of First Release Program Announcement - https://blogs.office.com/2015/12/08/more-services-join-first-release/
Individual User First Release Announcement - https://blogs.office.com/2015/05/05/manage-change-and-stay-informed-in-office-365/ and http://www.astaticstate.com/2015/05/office-365-first-release-for-specific.html