If three people eat together and have bread, they are required to recite the introductory Zimun before Birkat Ha’mazon. If there are fewer than ten people participating, they recite the simple text of the Zimun, which does not include the Name of Hashem, whereas if ten or more people participate, they recite "Nebarech Elokenu" and "Baruch Elokenu," including Hashem’s Name.
The Shulhan Aruch (Orah Haim 193:3) addresses a case which might at first not seem very relevant nowadays, but is actually important in terms of practical Halacha – the case of people who are riding together on animals and eat as they ride. If three people ride together on horses, for example, and they eat bread at the same time while riding, then they do not recite a Zimun before Birkat Ha’mazon. Since they eat while they are moving, the Shulhan Aruch writes, they are not considered as having assembled together for a meal, and so there is no requirement of Zimun. If, however, they stopped the animals before eating, and they ate while stationary, then they recite a Zimun even though they are each on a separate animal.
Hacham Bension Abba Shaul (Israel, 1924-1998) applied this Halacha to the case of people who are in a vehicle and eat together during the trip. If the car stops while they eat, then certainly they recite a Zimun, just as in the case of people traveling on animals who stop to eat. However, if the people in the vehicle eat while the vehicle is driving, then it is uncertain whether they are considered formally assembled such that they recite a Zimun. Therefore, Hacham Bension writes, if there are fewer than ten people driving together, they recite a Zimun. Since the Name "Elokenu" is not mentioned in the Zimun when there are fewer than ten men, there is nothing lost by reciting a Zimun. And if there are ten or more men who eat together in a moving vehicle, like on a tour bus, then they recite a Zimun without "Elokenu," due to the uncertainty as to whether they are considered as having eaten together.
Interestingly, however, Hacham Bension distinguishes in this regard between vehicles and airplanes. When people eat together on a boat, he writes, they recite a Zimun even though the boat was moving when they ate, because it is the water that carries the boat. Since the boat does not move on its own, it not considered "moving" with respect to this Halacha, and therefore, the people who eat together – such as in the dining room of a cruise ship – recite a Zimun, and if they are ten people, they include "Elokenu." Hacham Bension asserts that an airplane resembles a boat in this regard, and is considered stationary. Therefore, in his view, if ten or more people eat together on an airplane, they recite Zimun with the word "Elokenu."
Others disagree. Rabbi Moshe Ha’levi (Israel, 1961-2000), in his work Birkat Hashem, distinguishes between the case discussed by the Shulhan Aruch, of people riding on animals, and people traveling in a vehicle or plane. In a vehicle or airplane, the people are together within the same walls and under the same roof, and so it makes no difference whether they are moving or stationary while they eat. People eating while riding animals are not considered to be eating together because they are out in the open. But in a car, bus or airplane (or in a cruise ship), the people are all together in the same room, so-to-speak, and so they recite a regular Zimun. This is also the ruling of Hacham David Yosef, in his Halacha Berura.
Every person should consult with his Rabbi for practical guidance.
Summary: If a group of men eat together while riding in a vehicle, such as in a tour bus, then according to some opinions, they do not include "Elokenu" in the Zimun before Birkat Ha’mazon, even if they are ten or more men, unless the vehicle was stationary while they were eating. Others disagree, and rule that "Elokenu" is recited even in such a case. On a boat or airplane, "Elokenu" is recited according to all opinions.
