I know that this might be a bad example but we got Akechi’s social link before he joined. Maybe Kasumi will be a similar situationAkechi's auto-ranked though so we would for sure hit the rank to unlock all his party goodies, though. I sure hope Kasumi's doesn't auto-rank because that's gonna be P4G anime levels of shoehorning in the new girl.
Akechi's was forced progression so you'd be pretty much guaranteed to use him when he joined up and would theoretically take greater note of him than they would any other late joiner, who would naturally tend towards neglect on account of them being behind on social link bonusesAt the very least can we please get Rank 1 without having to clear their dungeon? Baton Pass is the #1 reason I ignore new party members.