Translation:Mishnah/Seder Zeraim/Tractate Berakhot/Chapter 7/3
How do we say Zimmun? With three, he says "Let us bless". With three in addition to himself, he says "Bless." With ten, he says "Let us bless our God." With ten in addition to himself, he says "Bless." The same for ten as for one hundred thousand.
With a hundred, he says "Let us bless YHVH our God." With a hundred in addition to himself, he says "Bless." With a thousand, he says "Let us bless YHVH our God, God of Israel." With a thousand in addition to himself, he says "Bless." WIth ten thousand, he says "Let us bless YHVH our God, God of Israel, God of the hosts, enthroned on the Cherubim, for the food we have eaten." With ten thousand in addition to himself, he says "Bless."
In the way he has blessed, so they respond after him: "Bless YHVH our God, God of Israel, God of the hosts, enthroned on the Cherubim, for the food we have eaten."
Rabbi Yose the Gallilean says, according to the size of the assembly do they bless, as is said "In assemblies bless God, YHVH from the source of Israel." [Ps. 68:27]
Rabbi Akiva says, What do we find in the synagogue? The same for many as for few: he says "Bless YHVH". Rabbi Yishmael says, "Bless YHVH the blessed."
Explanation:
With three in addition to himself. The minimum number for Zimmun is three, so together they formally open the blessing by saying "let us bless". If there is a fourth, he can address the three, and state [in the imperative] "bless." Similarly, since there need to be ten to use God's name in the zimmun, if there are only ten, together they say, "let us bless our God"; but if there is an additional person, he can address the ten and state [in the imperative] "bless our God".
The same for ten as for ten thousand. This is the normative halacha: The language of the Zimmun for ten does not change no matter how many additional people are saying it.
With a hundred The next paragraph describes the position of Rabbi Yose the Gallilean. His reasoning appears afterward. With a hundred in addition to himself Rabbi Yose makes the same distinction made by the previous (anonymous) source: if there is more than the minimum number, the language changes from "let us bless" to the imperative "bless".
Rabbi Akiva says Rabbi Akiva gives the rationale for why the blessing should not change, no matter how much the numbers increase beyond ten. He draws a parallel to the language of the synagogue service --no matter how large the congregation, the Borchu (formal call to worship, analgous to the zimmun) is always the same -- "Bless YHVH."
Rabbi Yishmael says Rabbi Yishmael agrees with Rabbi Akiva, but notes that the precise formula for the synagogue Borchu is "Bless YHVH the blessed".