70 points in liking you.
Each time you ask them about Fun, Prize Money, Audition, or Pet Peeve, you gain two points.
Certain choices during encounters can also cause you to gain or lose points with them: praising Owen when you first meet him makes him like you more; during the early argument between Geoff and Bridgette, you'll gain points with who you side with, and lose them with the other; insulting Heather's chest when you first meet her will lose points; and of course, if you're on the Bass team, you can get points from ALL the girls by jumping naked in the first challenge, with a little extra from Eva for not making her do it.
You can check how much everyone likes you in the (i) menu of the UI.
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Click on a character to see how much they like you.
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As for nude hot tubbing, I've managed to get it with all the girls available for it by maxing out their attitude towards me.