Sefer Ĥaredim (12:18) states that not only the kohanim fulfill a mitzva by blessing the congregation, but the Jews who stand before them in intent silence and respond “amen” also participate in the fulfillment of this biblical commandment.
When the kohanim perform Birkat Kohanim, the congregation must stand in front of them, as it is written: “This is how you shall bless the Israelites. Say to them” (Bamidbar 6:23). The Sages interpret this to mean that Birkat Kohanim must be performed in the same way that people talk to their friends – by standing face to face and speaking aloud so that all those receiving the blessing can hear them. Although technically the audience may sit, it has become customary that all stand for Birkat Kohanim. Still, an ill or weak person who has difficulty standing may sit (MB 128:51; Tzitz Eliezer 14:18).
One who cannot come to the synagogue due to matters out of his control – for instance, one who must go to work – is still included in the berakha. Similarly, women and children who are not required to come to the synagogue are included in the berakha. Only men who can go to the synagogue but neglect to do so are excluded from the berakha.
One who comes to the synagogue but stands behind the kohanim during Birkat Kohanim is not included in the berakha. If he is standing directly alongside them, he must turn toward them to be included in the berakha. Those sitting in the first pews of the synagogue must measure where they are in relation to the kohanim. If they are in front of them or even directly to the side of them, they may remain in place and turn toward the kohanim. However, if they are slightly behind the kohanim, they must move elsewhere for Birkat Kohanim. The same ruling applies to women in the women’s section at the sides of the synagogue (SA 128:24).
Anyone standing in the synagogue facing the kohanim is included in the berakha. Even if there are taller people or pillars in front blocking the kohanim from view, she is indeed included in the berakha since she is on the side that faces the kohanim. However, one who turns her back to the kohanim is not included.
If one is in the middle of the Amida when the kohanim start to bless, she waits a bit and listens to Birkat Kohanim, which is a biblical commandment, and then she continues praying. However, she must take care not to answer “amen” to Birkat Kohanim, so as not to interrupt her Amida (Peninei Halakha: Prayer, ch. 20 n. 2).