Most of the time (100% for me so far) you just have to have pre-check for whomever is buying the tickets. As long as they're all on the same reservation ID, the pre-check will get applied to the entire group. That's how I got my first taste of pre-check, when my sister-in-law bought tickets for our entire family. I ended up buying it for myself, and now my wife & kids get the stamp as well when we're traveling together.
They'll tell you it's not guaranteed (and it's technically not even guaranteed that you'll get PreCheck even if you've got PreCheck yourself) but I've also had a 100% success rate with buying tickets and having PreCheck applied to all of them.
