"No, we only support individual accounts during Project Fi's Early Access Program. If multiple people on your current wireless plan are interested in joining our early access program, each user is required to have a Project Fi account."
Given that you only pay for the data you use, data sharing is not needed
It's $10 per Gigabyte, no discounts, that's it (but the billing is in some fraction of a gigabyte).
It also only supports the Nexus 6, so I guess you won't want to switch 3 lines over.
from the FAQ:
Does Project Fi have family plans or other plans for multiple users?
No, we only support individual accounts during Project Fi's Early Access Program. If multiple people on your current wireless plan are interested in joining our early access program, each user is required to have a Project Fi account.