Of course I booked for family members, and only after they asked me to book.
They didn't have their ID documents at hand though, so I created their accounts with dummy data. We'll see how that will go, but I've submitted passport information anyhow.

I was really tempted to sign up my dog, but I wasn't sure if 5-year-olds can be Iberia Plus members.

I doubt that there will be a lot of fake accounts. Most of us probably know enough people they could convince to jump on this promotion.