Project Fi has been around for a good little while now, and as a typical Google product, it has seen some slow changes to make it an even better service. Project Fi runs off of Sprint, T-Mobile and US Cellular as well as public hotspots. The service will intuitively switch between these networks when needed, making sure you have the best signal and data speeds possible, at all times. It will also automatically switch over to a public hotspot and flip on the built-in VPN, so you can continue using your phone without using data. Plans start at $30/month for 1GB of data and each gigabit is another $10/month. It's not exactly family friendly, as far as pricing goes, but that may soon change.
In the last update to Project Fi, which is K.2.7.13, there is code that shows Project Fi is indeed preparing for group plans for the service. There will be an owner that can adjust everything on every line on the account. There will also be usage alerts so you can see when other users are close to their data limit. The owner would also be able to change the data allowance for each line, and this would be an individual change. Therefore you could make one line have 2GB of data, and another line have 5GB, etc. This update will also likely make it easy to cancel service for one of the lines on the account. Instead of with typical carriers where you need to call in and have it done. With Project Fi, you'll be able to do it through the app, making it a simple and painless change.
It's important to note here that these group plans have not been announced by Google yet. But the code is there in the latest version of Project Fi, which means they are likely looking to add group plans in the near future. The only question is when. It shouldn't be much longer, likely just a bit more testing is needed before they go live. Project Fi, as a reminder, only works with the Nexus 6P, Nexus 5X and Nexus 6 for now. So if you are interested in using or switching to Project Fi, you'll need one of those three smartphones.