Beitzah, Chapter 3, Mishnah 6. The Mishnah opens by stating: "Ein nimnin al behemah lechatchilah b'Yom Tov".
"Nimnin" means that a group of people approaches the butcher and asks to divide an animal among themselves, each one being counted in and taking a portion of it. The Mishnah teaches that one may not arrange such an arrangement in the first instance on Yom Tov, agreeing to purchase an animal for a given price and settling each person's share of the cost.
The Mishnah continues: "Aval nimnin aleha erev Yom Tov, v'shochtin u'mechalkin beineihen" - it is permitted to divide the animal among the group of people before Yom Tov, and on Yom Tov itself to slaughter it and distribute it according to what was decided on the eve of Yom Tov. The prohibition applies only to making the financial arrangements on Yom Tov itself.
The dispute between Rabbi Yehudah and the Sages regarding weighing meat:
"Rabbi Yehudah omer: shokel adam basar k'neged ha'kli u'k'neged ha'kufitz" - one may not use ordinary weights to weigh items on Yom Tov, since this is a weekday activity and is not permitted. However, if a person knows the weight of a particular utensil, or of the kufitz (the butcher's cleaver), he may weigh the meat against it and see whether it matches that weight. After Yom Tov he can weigh that item, and then calculate how much that quantity of meat weighed.
"V'chachamim omrim: ein mashgichin b'kaf mozna'im kol ikar" - the Sages disagreed and ruled that one does not look at a scale at all on Yom Tov, and one may not use it in any manner whatsoever.
The Gemara explains the emphasis of the phrase "kol ikar": even if a person has no intention of using the pans of the scale for weighing, but only to hang the meat on them in order to keep it away from mice or from other things - it is still not permitted, because he appears to be using a scale for weighing on Yom Tov.
In summary: in this Mishnah we learned that one may not arrange a joint purchase of an animal in the first instance on Yom Tov, but rather on the eve of Yom Tov, and on Yom Tov itself the animal is slaughtered and divided as was agreed beforehand; and Rabbi Yehudah and the Sages disagreed about weighing meat - Rabbi Yehudah permits weighing against a utensil or against the cleaver, while the Sages forbid looking at the pan of a scale at all, even when it is not for the purpose of weighing.