It was probably done that way to give Michelle/Janet a more prominent place on the poster but still the poster is imbalanced with 3 people on one side and two on the other.
Well, there's Janet/Hank and Bill Foster/Luis, so that comes across to me as two pairs of allies, as in the trailers. They did add in the other guy (who is Sonny Burch, I guess?), but he's not featured prominently at all, so I would think that it's to create the slightly asymmetrical visual.
I doubt that it's supposed to be a group of bad guys, tbh.