Berachos, Chapter 7, Mishnah 2. In this Mishnah we will learn two matters: who are those who do not join together for a zimun, and what amount of eating obligates a person in Birkas HaMazon and allows him to join a zimun.
The words of the Mishnah: "Nashim va'avadim uketanim - ein mezamnin aleihem" - women, slaves and minors cannot be counted as part of a zimun.
The Gemara's explanation of the three categories:
Women: they may make a zimun on their own.
Slaves: they may make a zimun among themselves, but they do not join together with women for a zimun.
Minors: since they are not obligated in the mitzvos, they cannot make a zimun even among themselves.
It is worth noting that in a zimun of women or of slaves the Name of the Holy One, Blessed is He, is not said, and even if there were ten of them, this is not considered a minyan.
Up to how much do we make a zimun?
Here the Mishnah clarifies what amount of eating is required in order to become obligated in Birkas HaMazon and to join a zimun. On this point the Tannaim disagree:
The first Tanna: "Ad kezayis" - the measure of a kezayis, which is about half the size of an ordinary egg.
Rabbi Yehudah: "Ad kebeitzah" - only when the eating reaches the measure of an egg.
The root of the dispute - "Ve'achalta vesavata":
Their disagreement stems from how they understand the verse "Ve'achalta vesavata uveirachta" - "And you shall eat and be satisfied and bless":
According to Rabbi Yehudah: both conditions - eating and being satisfied - refer to the food. Ordinary "eating" in the Torah has a measure of a kezayis, but the added word "vesavata" teaches that a satisfying amount is required, and a person is not satisfied with less than a kebeitzah. It emerges that eating is considered eating with a kezayis, but it is not considered satisfaction until a kebeitzah.
According to the first Tanna: "Ve'achalta" refers to eating, and "vesavata" refers to drinking. Therefore an amount of only a kezayis of food is sufficient.
In summary: in this Mishnah we learned that women and slaves do not join a zimun together with others, although they may make a zimun on their own and without mentioning Hashem's Name, and that minors do not make a zimun at all because they are not obligated in the mitzvos. We also examined the dispute of the Tannaim regarding the amount of eating - a kezayis according to the first Tanna and a kebeitzah according to Rabbi Yehudah - and its root in the understanding of the verse "Ve'achalta vesavata uveirachta".