Often at a Shabbos seuda, the baal habayis (head of the household) will make a hamotzei and have in mind everyone at the table. Generally everyone washes and when everyone returns to the table the hamotzei is made. If however, one of the people just washed and has not had a chance to make an al netilas yadayim and dry his hands at the time the head of the household is ready to make the hamotzei, he may still listen to the birchas hamotzei, then make an al netilas yadayim dry his hands and then eat the challah without reciting a new bracha (Shulchan Aruch 167:7).
The Mishna Berurah (167:45) takes it one step further and says even if one has not yet washed his hands, he may b’dieved first listen to the birchas hamotzei then wash his hands, make an al netilas yadayim and eat the challah without reciting a new hamotzei relying on the hamotzei of the baal habayis. The reason is that washing, making an al netilas yadayim, and drying ones hands are not considered a hefsek between the birchas hamotzei and eating the challah being that they are considered necessary for the meal.
However, if one is in middle of reciting an al netilas yadayim at the time that the baal habayis is reciting the hamotzei, then he is not yotzei with the baal habayis’s bracha being that he was speaking during his bracha. In such instances that person would have to recite a new hamotzei on the challah (Mishna Berurah 167:45).