Hacham David Yosef, in his work Halacha Berura (Siman 170; listen to audio recording for precise citation), codifies a number of Halachot pertaining to proper conduct at meals. He writes that a guest may not share the food served to him by the host to other people, even members of the host’s family. If, for example, a guest wants to share his food with the host’s children, he must first receive the explicit consent of the host. The reason, as Hacham David explains, is that the host might not have enough food for everyone. If the host suspects that the guests did not receive enough food, he will feel embarrassed, and the guests must therefore not share their food with anybody.
However, a guest may share his food with the host’s family members if he sees that there are plentiful amounts of food available for both the family and guests. Likewise, if a guest sees that some food has been left over after the meal, he may share some of the food served to him with the host’s children.
Children who are supported by their parents may share some of their parents’ food with a poor person in need, even without receiving the parents’ explicit consent. Hacham David explains that nowadays it can be assumed that the parents would agree to share their food, even if they are not consulted.
It is proper for a host to personally place the food on the guest’s plate and to pour his beverage for him. This applies even if the host is a distinguished person. As the Magen Abraham (Rav Avraham Gombiner, Poland, 1637-1683) notes, our patriarch Abraham personally tended to his guests despite his stature of distinction. His example teaches that regardless of a person’s high stature, he should personally serve his guests.
The Sages teach that the "Nekiyeh Ha’da’at" ("pure-hearted") in Jerusalem would ensure never to participate in a meal without first checking who else would be attending the meal. The reason, as the Sages teach, is that "it is shameful for a Torah scholar to sit together with an Am Ha’aretz [boorish person] at a meal." Some authorities maintain that this refers only to Torah scholars sitting at the same table with an Am Ha’aretz; a Torah scholar may attend an affair where an Am Ha’aretz is present, so long as he sits at a different table. Rav Haim Palachi (Turkey, 1788-1869), however, disagrees, and maintains that a Torah scholar may not attend a function where an Am Ha’aretz is present even if they sit at different tables.
According to some opinions, this Halacha applies even at a "Se’udat Misva" (meal involving a Misva), such as a Berit Mila or Sheba Berachot. Even at these kinds of meals, a Torah scholar should not sit in the company of an Am Ha’aretz. However, Hacham David notes that the common practice is to allow Torah scholars to attend a "Se’udat Misva" even if there will be an Am Ha’aretz in attendance. Hacham David adds that if the scholar’s presence would serve an important purpose, such as at fundraising functions and the like, then to the contrary, it is a Misva for him to attend, despite the presence of Ameh Ha’aretz. Not to mention, he writes, that if the Ameh Ha’aretz have come to listen to the Rabbi speak, and not to insult or deride him, then certainly he fulfills a great Misva by attending the function.
In any case where Halacha forbids a Torah scholar to attend a meal with an Am Ha’aretz, he must ensure not to disclose the reason for his refusal to attend, in order not to offend the Am Ha’aretz.
Finally, the Be’er Heteb (Rabbi Yehuda Ashkenazi of Frankfurt, 1730-1770) writes (170:16) that one should avoid, when possible, sitting down to a meal with gentiles. This ruling is based on a passage in Pirkeh De’Rabbi Eliezer that strongly condemns participating in meals with non-Jews.