The notifications read “you’re invited” and encourage users to “save bandwidth and browse more securely” by tapping the big blue “Enable” button below the alert’s text. If you choose to opt in, you’re taken to a Settings interface where you can toggle “Reduce Data Usage” on or off, and presumably later view the ongoing savings Google provides. Unfortunately, without the invite, there doesn’t appear to be a way to toggle the feature on by yourself. Similarly, if you initially decline the invite, you can’t seem to pull up the option a second time.
Unfortunately, as of yet there does not appear to be a way to request an invite.
Check out some screenshots below...