You can add up to five additional people to your existing Project Fi account, and manage their data and billing in one central place. Additional group members join your plan at $15 per month (versus $20 for the main line) and then you buy data in the $10/gig buckets and that includes the refund for unused portions.
I've been on Fi for a while now and love it. I'm nearly always around WiFi and my average bill is ~$27. I just ordered a Pixel XL but with it's release the Nexus 6P is down to $399 and the 5X is down to $199. It might not be the best plan for everyone. If you're a heavy data user or mobile streamer something like T-Mob's plan where that's exempt might be more economical. You'd have to analyze your data usage to know for sure but I can say that so far Fi has been great.