For one day only, all you have to do to get a Project Fi invite is ask.
Google is opening signups for experimental mobile phone service available to anyone who asks today in celebration of the release of the Nexus 5X, the LG-built device that proves smartphones don't need to be expensive to be good. (Typically, signing up for Project Fi requires receiving an invite from Google, then waiting for approval.) The larger Nexus 6P should become available shortly as well.
Announced this past April, Project Fi uses a combination of Wi-Fi and cellular service (Google leases access from Sprint and T-Mobile) to make calls and access data. By default, Project Fi will use an available Wi-Fi hotspot for voice and data, but will fall back on cellular in areas where there's no Wi-Fi. One of the biggest perks of Project Fi is the ability to choose a data plan, then have any unused data roll over to the next month. Plans start at $20 per month, and this include unlimited voice and data; additional cellular data costs $10 per 1GB.
Right now, Project Fi is only compatible with 2014's Nexus 6 and the new Nexus 5X and 6P. You may want to temper your expectation, however, with early reviews of the service being mixed: CNET was disappointed in 4G data speeds while PC Mag dinged the service for spotty coverage and the lack of family plans.