אורח חיים קצ״חOrach Chayim 198

Se’if 1א׳

שְׁלֹשָׁה שֶׁאָכְלוּ וְהֵם מְבָרְכִין, וְנִכְנַס אֶחָד שֶׁלֹּא אָכַל, אִם נִכְנַס כְּשֶׁאוֹמֵר הַמְבָרֵךְ נְבָרֵךְ שֶׁאָכַלְנוּ מִשֶּׁלּוֹ, עוֹנֶה אַחֲרָיו: בָּרוּךְ וּמְבֹרָךְ שְׁמוֹ תָּמִיד לְעוֹלָם וָעֶד; וְאִם נִכְנַס כְּשֶׁאֲחֵרִים עוֹנִים בָּרוּךְ שֶׁאָכַלְנוּ מִשֶּׁלּוֹ, עוֹנֶה אַחֲרֵיהֶם אָמֵן. הַגָּה: וְכֵן בְּכָל הַבְּרָכוֹת שֶׁאָדָם שׁוֹמֵעַ, חַיָּב לַעֲנוֹת אָמֵן (טוּר וּבֵית יוֹסֵף בְּשֵׁם בה"ג וּסְמַ"ג) וְאִם הֵם עֲשָׂרָה, אוֹמֵר: בָּרוּךְ אֱלֹהֵינוּ וּמְבֹרָךְ שְׁמוֹ תָּמִיד לְעוֹלָם וָעֶד; וְהוּא הַדִּין אִם הָיָה שָׁם כְּשֶׁגָּמְרוּ מִלֶּאֱכֹל וְלֹא אָכַל עִמָּהֶם, כָּךְ הוּא עוֹנֶה אַחַר הַמְבָרֵךְ וְאַחַר הָעוֹנִים.

Three who ate and are blessing, and one who did not eat came in: if he came in when the one who blesses says "Let us bless Him of whose bounty we have eaten", he answers after him, "Blessed and blessed be His name continually for ever and ever"; and if he came in when the others are answering "Blessed is He of whose bounty we have eaten", he answers "amen" after them. Hagah [Rema]: And likewise in all the blessings that a man hears, he is obligated to answer amen (Tur, and Beit Yosef in the name of Behag and Semag). And if they are ten, he says, "Blessed is our God and blessed be His name continually for ever and ever". And the same is the law if he was there when they finished eating and did not eat with them: so he answers after the one who blesses and after those who answer.