The Mishnah states that three people who ate together must make a communal invitation for the blessing after meals.
The Talmud Yerushalmi raises a question. In one place, it says that three people who ate together may not separate. In another place, it appears to say that they may separate. What is the question? These are two different expressions. One statement says that they may not separate, while the other uses different language. Why, then, does the Talmud Yerushalmi ask a question?
Therefore, Tosafot has to explain what the question of the Talmud Yerushalmi actually is. When it says that if they finished the meal together, it sounds as though one person could finish his meal before his friends and recite the blessing after meals on his own. The requirement applies only when they finish together; in that case, they must make the communal invitation. But if one person finishes first, he can recite the blessing after meals on his own.
However, the expression “they are obligated to make a communal invitation” implies that the obligation applies to all of them. They are not permitted to finish the meal independently, one without the others. This is the question of the Talmud Yerushalmi.
There are three formulations in the language of the Talmud Yerushalmi, and Tosafot explains that they reflect a dispute between two amoraic authorities: “Here we are speaking at the beginning, and here we are speaking at the end.” The question is: What is meant by “at the beginning,” and what is meant by “at the end”?
One authority says that “at the beginning” means that they initially intended to eat together. If they set their minds to eat together, that is considered the beginning. If they ate an olive’s measure together, that is considered the end.
The other authority says the opposite: if they ate an olive’s measure together, that is considered the beginning; if they later decided to eat together, that is considered the end.
Which answer is the primary one? Tosafot ultimately follows the first approach, based on the principal wording. This means that they sat down together, but one person ate on his own. Even though they finished the meal together, they must make the communal invitation. However, in that case, he is able to separate himself and recite the blessing after meals on his own.