Just did this for my parents onboard two weeks ago on the app in the part where it says folio, packages, and prizes or something along those lines. For a 5 day cruise, princess plus was $300 each and they had $200 obc each. I was a little worried all of their obc would apply because the app makes you purchase for both people from one person's account. You'll see the credits applied in your journey account. For some reason, they got a $10 discount for the bundle! Don't know how that happened but it was a nice surprise (They paid $180 total after obc for both) I was able to see it was active because now the part where it says packages showed how many of each part of the package they had redeemed aka 0/15 drinks redeemed, 0/2 premium desserts, etc. I don't know if it would make a difference if you sign up with the crew. They would be probably just tell you how to do it in the app unless you're at guest services. I don't think they get any incentives from your buying it.