The Shulhan Aruch (Orah Haim 195) writes that if people are seated at different tables, they nevertheless join together to form a Zimun if two conditions are met: they can see each other, and they intended to eat together. As long as these two conditions are met, they can form a Zimun even though they sit at separate tables. The Shulhan Aruch adds that even if they cannot see each other, they can join together for a Zimun if they share the same waiter, who is Jewish. If the same waiter serves the people at both tables, then even if they cannot see each other, they join together for a Zimun if they planned to eat together.
Hacham Ovadia Yosef adds that if there are fewer than ten people, then they can recite a Zimun even if they did not plan to eat together. As long as they can see each other and they ate at the same time, they can recite a Zimun. When fewer then ten people recite a Zimun, Hashem’s Name is not mentioned, and so there is greater room for leniency. As such, people can join together even if they sit at separate tables and did not plan on eating together, as long as they ate at the same time – either at the beginning or at the end – and they can see each other (or have the same waiter).
Hacham Bension Abba Shaul (Israel, 1924-1998) ruled that when it is obvious that a group of people intended to eat together, this does not have to be explicitly stated for a Zimun to be recited. At a wedding, for example, it is obvious that the guests all intend to eat together in the banquet hall. Therefore, they all join together for a Zimun – with Hashem’s Name, as they are more than ten people – even they did not say explicitly that they planned to eat together. Another example, as noted by the Minhat Yishak (Rav Yitzchak Yaakov Weiss, 1902-1989), is a yeshiva dining room, where the students eat meals. Since it is clear that they plan to eat together in the dining room, they all join together for a Zimun even though they did not specifically plan to eat together.
Summary: A group consisting of fewer than ten people can join together for a Zimun even though they sit at separate tables, as long as they eat at the same time, either at the beginning or at the end, and they can see each other. Even if they don’t see each other, it is enough that they are served by the same waiter, who is Jewish. A group of more than ten people sitting at different tables joins together for a Zimun with Hashem’s Name only if they can see each other (or have the same waiter) and they specifically planned to sit together. If it was self-evident that the people intended to eat together, such as in the case of a wedding or a yeshiva dining room, then this does not have to be explicitly stated.
