Pick up a nice little bill credit for referring friends to join Project Fi.

Google likes to give you little tips about your account when you open up the Project Fi app, and now you're going to start seeing a reminder to "refer a friend." It's really simple: just tap "refer a friend" in the app, and you'll see a unique code. When someone signs up using that code, both you and the person you referred get $20 toward your account once the new account has been active for a month.

You can now get $20 for bringing friends to Project FiYou can now get $20 for bringing friends to Project Fi

The referral system will be active until January 11, and if you send out a code before then the person will have to activate by that date in order for you both to receive the account credit. The one caveat here, it seems, is that additional members on a group plan are not eligible to be referred (which kind of makes sense).

I'm not sure how many people are going to be swayed to trying Project Fi by the promise of a whopping $20 when the likes of the big carriers in the U.S. are offering hundreds in switching bonuses. But this is of course yet another clear point that Google is working to grow the Project Fi subscriber base. Add to it the big announcement recently that there are new group plans, and you can be sure that Project Fi is growing.

So if you're going to be giving Project Fi a try this holiday season, be sure to ask around if anyone you know has a code — you might as well save a few bucks on your first bill by getting a referral.