Today, Google announced its Project Fi Group Plan for up to six lines per account. Via its Fi Basics plan, which includes unlimited talk and text, it costs $20 for the first line and $15 for each additional line. That's a savings of $5 for each additional line against using separate accounts per line. Data costs an additional $10/GB; however, any unused data is prorated and refunded back to you in dollars and cents. Click here for more info.
Note that this service is currently only compatible with Google Pixel, Nexus 6P, Nexus 5X, and Nexus 6 phones.