בְּֽ֭מַקְהֵלוֹת בָּרְכ֣וּ אֱלֹהִ֑ים יְ֝הוָ֗ה מִמְּק֥וֹר יִשְׂרָאֵֽל׃
"Dans vos groupes, Bénissez Dieu! Bénissez le Seigneur, [vous tous, issus] de la source d’Israël!
Rabbeinu Bahya
A Midrashic approach: the word ויאמרו refers to infants reciting the שירה. The word לאמור is a reference to the unborn children within the wombs of their mothers. This is in line with Sotah 30 where we are told: “how did the Israelites recite the שירה?” Answer; a slightly older infant while he was lying on his mother’s knees; a still younger infant while he was nursing at his mother’s breast. As soon as the infant lying on his mother’s knees became aware of the presence of the Shechinah he raised his neck, whereas the suckling infant let go of his mother’s nipple and both recited: זה אלי ואנוהו, “this is my G’d let me exalt Him.” The Midrash is based on Psalms 8,3 מפי עוללים ויונקים, “from the mouths of infants and sucklings.” Even embryos in the wombs of their mothers said the שירה as we know from Psalms 68,27: “in assemblies bless G’d, the Lord, O you who are the fountain of Israel.” [The words ברכו...ממקור were a call to unborn babies. compare Midrash Tehillim there. Ed.]
Bartenura
במאה אומר נברך לה' אלהינו – The concluding phrase of the Mishnah is all [according to] Rabbi Yosi HaGelili, who said that according to majority of the community, they recite the [appropriate] blessing, as it says, “Psalms 68:27): “ln assemblies bless God,[ the LORD, O you who are from the fountain of Israel].” But the Halakhic decision is that from three [adult males] until ten [adult males] but the tenth is not included, the person who blesses (as the leader) recites: “Let us bless [Him] that we have eaten from what is His, and everyone answers: “Blessed that we have eaten from what is His, and through His Goodness we live.” And ten or from ten and above, the person leading the blessing states, “We give praise to God that we have eaten from what is His and through His goodness we live.
Bartenura
וכן ארבעה וכן חמשה – The “three” [of that group of four or five] (cannot separate) to recite ברכת המזון for themselves and the individual should separate himself off from them, for he also has an established obligation for the appeal to participate in the group recitation [of ברכת המזון], it says Psalms 68:27), “In assemblies bless God, [the LORD, O you are from the fountain of Israel].” And the determined Halakhic practice is that from three individuals until ten individuals, but not including ten, the person reciting the blessing says, “Let us thank Him whose food we have eaten/נברך שאכלנו משלו,” and everyone responds, “ברוך שאכלנו משלו ובטובו חיינו/Blessed is [He] whose food we have eaten and through whose goodness we live.” And with ten or more than ten people, the leader says, “נברך לאלהינו שאכלנו משלו/Blessed is our God whose food we have eaten.” And everyone answers: “ברוך לאלהינו שאכלנו משלו ובטובו חיינו/Blessed is our God whose food we have eaten and through whose Goodness we live.”