I haven't written a CG unlock guide, but I can point you in the right direction. For Griffin, go hang out with Asher at the club when you pick who you want to talk to after Will's class. That choice will also unlock CGs for Asher and Mafia/NC17. For the duos & trios, you need to have expressed attraction to everyone included in those CGs for you to be able to unlock them. The options to unlock them are during the fantasy sequence after orientation, so before then make sure you picked a "he's hot" choice for the guys in these groupings: Matt/Leo, Owen/Julian, Will/Tomas, and Asher/Mafia/NC17.
The last images for everyone else are either unlocked during the fantasy sequence after orientation (so just pick their option there) or during one of the hangout sessions after Will's class if they're one of the eight hangout options (Alex, Asher, Dominic, Isaiah, Marco, Matt, Owen, & Riley). Those hangouts have two CGs each for all eight guys.
Hope that helps you track them down! If there are any others missing, let me know the character and where on the grid the missing image is and I can tell you how to unlock it.