You have a thinking error. The MC, Sage and Quinn are planning the Halloween party and it will be at DIK. Ergo, they will definitely be partying at the DIK. If the MC is partying at the DIK, all the LI's could be at the DIK if the MC wasn't screwing with them.
Example.
My MC is with Josy and Maya and good friends with Jill, Sage, Isabella. What obstacle could keep Jill and Isabella from coming to the DIK Halloween party? None.
Theory of why all LIs could come to the DIK Halloween party. The DIK mansion becomes a LI battleground for the MC. But there are complications because the MC now looks like Tommy. Hahaha.