Replaying the game, I thought that
dalli_x was really right about the fact that the guitar could have been stolen by the Diks. But I don't think their goal was to bring MС and Sage together, and everything is much simpler.
It always confused me why Tommy and the others were ready for MС's visit, it was as if they were waiting for him. He hadn't applied to join Diks, but for some reason, Quinn did an offer about restaurant to MC and she organize a shower prank. Apparently, the fact that MC immediately quarreled with Chad and openly molested Sage was the main reason to prepare MC for joining Diks.
As we remember, they had only three newcomers, Derek apparently immediately joined and told about MC. Sage said that a close friend gave her the guitar, it could have been Quinn, not Chad, as many thought. Maybe Tommy broke into MC's room, beat up Troy and took the guitar? He seemed to know about Troy's homosexuality, because Troy did not immediately tell, and just drove him away.
I really think that some of the side intrigues and riddles of the game will not be revealed, such as the intrigue of who stole the guitar MC. This is the beginning of the game, but I think Pink himself forgot about it. The problem with Cathy is too significant to ignore, and I hope that it will be revealed.