With the first release you’ll get product updates as they become available in an early preview stage. Using the standard release you will receive product updates when they are in a final stage and generally available.
By activating the «Frist Release» option in your Office 365 you will receive preview releases of new products and services. To do so:

Sing in to your Office 365 tenant and navigate to the Office 365 admin center
Click on Service Settings > Updates on the left
To enable first release choose Entire organization or Select people. Note: Some of the previews require to enable first release for the entire organization.

By choosing «some people» you can add users individually by selecting them in the next step or you can bulk add users by uploading a text file.