The Mishna on today’s daf distinguishes between a case where several people were sitting to eat, which is not a joint meal, where each and every diner recites a blessing for himself; and a case where they were reclining on divans, which renders it a joint meal, and one recites a blessing on behalf of all of them.
The Gemara infers:
If they reclined, yes, it is considered a joint meal; if they did not recline, no. And the Gemara raises a contradiction: Ten people who were walking on the road, even if they are all eating from one loaf, each and every one recites a blessing for himself. If they sat to eat, even if each and every one is eating from his own loaf, one recites a blessing on behalf of them all as it is considered a joint meal. In any case, it was taught: If they sat to eat, even though they did not recline. Apparently, sitting together is enough to render it a joint meal and reclining is not required.
Rav Naḥman bar Yitzḥak said: With regard to those walking along the road, it was in a case where they said: Let us go and eat in such-and-such a place. Since they designated a specific location to eat together in advance, it is considered a joint meal.
In talmudic times, the custom was to partake of significant meals while reclining, not sitting upright. This was the custom of the wealthy, free men, who had the ability to leisurely relax and carry on conversations during the meal.
Tosefot Rabbeinu Yehuda HaḤasid explains that the term heisevu – “they reclined” – both here and elsewhere (e.g. at the Pesach), is related to the word sivuv – “circle” – meaning that those reclining sat around a table or surrounding the surface where the food was placed. Based on that interpretation, individuals surrounding the bread, even when not reclining in the literal sense, are considered to have dined together, and one may recite a blessing on behalf of the others.