I would immediately chop the throttles, look over my shoulder in the direction they went overboard, mark the position on the plotter and turn on all the lights while ordering someone to toss in a lifering w/ strobe (not only for it's utility as a flotation aid, but as a visual reference of the approximate position the person went in, and a means to get a read on set and drift), sound 5 or more short and rapid blasts on the whistle, remind the pointer not to take his/her eyes of the MOB, execute the proper turn and downswell run to set up for a good approach, then recover the person either indirectly with a throw line, or directly alongside.
I've practiced that once or twice, too. Be more than happy to explain the idea behind any part of it.
But like any emergency on any boat, you can break it down into the three distinct parts of my 'dealing with it' theory: Identify the problem, Isolate the problem, Remedy/Repair/Deal With the problem.
Identifying the problem could be one of your crew shouting "MAN OVERBOARD, PORT SIDE!"
Isolating the problem could be "Hey guys, Bob here isn't going to take his eyes of the Man Overboard, meantime I need all of you to sit down in the seats and keep your eye on him, too. That way nobody else falls over while I maneuver the boat to pick him up, and nobody blocks Bob's view."
Remedy the problem would be picking him back up.