If you are uncertain when to head up, point the boat towards the MOB. If the sail flaps to leeward you are in the right place. If still powered up, bear away and try again. If you find you are head to wind, sail past and tack round to try again on the opposite tack.
Take great care not to capsize the catamaran after someone has fallen overboard. Follow the steps above, but gybe (verb) the catamaran and on the approach collect the person between the hulls by the cross beam.