>>103680184
They're really high up there, but I'm picking them in part because they are really close friends and will have really varied reactions, it's like a balanced meal.
Okayu will obviously enjoy it a lot, being a masochist, but will also needle the others. Korone will think it's a joke and go along with it, only to gradually discover it's not a joke at all. Mio will be shocked that you would do this to her and will try to appeal to your decency while also barely hiding that she's really getting into it. Fubuki will accept it unquestionably and give you pointers about what her colleagues did recently that really calls for a punishment. If Kurokami is there as a bonus (median gen has five people in it!) that's probably going to be the most fun, she would be totally and utterly pissed that you even dare to touch her and desperately hide how wet she's getting.
As for how, I'd line them up in a row leaning over something like the back of a sofa and go from butt to butt.