The company announced the news on Twitter saying, "The #Nexus5X ships today–so Project Fi is giving out instant invites for 24hrs to celebrate".
Project Fi is basically Google's own mobile network that works using a combination of T-Mobile, Sprint, and Wi-Fi. For $20 a month you get all the basics (talk, text, Wi-Fi tethering, and international coverage in 120+ countries), and then it's a flat $10 per GB for cellular data while in the U.S. and abroad. 1GB is $10/month, 2GB is $20/month, 3GB is $30/month, and so on. You also get credit for the full value of your unused data at the end of the month.
Project Fi works with the new Nexus 5X and Nexus 6P as well as the Nexus 6.
You can get an instant invite at the link below...