Gemara

(Beraisa - R. Eliezer ben Yakov): A guest may not give his portion to the host's children without permission.

Once, Ploni invited three guests at a time of famine. Each gave his meager portion to Ploni's son. Ploni saw his son eating one of them and holding the other two. He (picked up his son and) slammed him on the ground, and the son died. His mother saw, and killed herself. Also Ploni killed himself.

R. Eliezer ben Yakov: Three people died due to this (giving one's portion to the host's child)!

107b (Beraisa): One may not give bread to a waiter, whether he or host is holding the cup, lest a calamity result.

Kesuvos 61a (Rav Yitzchak bar Chananya): One may delay letting a waiter taste any food except for meat and wine.

(Rav Chisda): This applies only to fatty meat and old wine.

(Rava): It applies to fatty meat the entire year, and to old wine only in summer.

The rule is, anything with smell or pungency must be tasted.

Rishonim

Rif and Rosh (Kesuvos 5:25): One may delay letting a servant taste any food except for meat and wine. This refers to fatty meat the entire year, and old wine in summer.

Rambam (Hilchos Berachos 7:7): The waiter who stands in front of those seated to eat does not eat with them. It is merciful to put into his mouth from every Tavshil (cooked food), to calm his mind.

Rambam (10): Guests may not give anything from what is in front of them to host's son or daughter, lest the host be embarrassed, for he has only what he put in front of them, and the children will take the portions and go.

Magen Avraham (170:23): The Rambam forbids, lest the host have nothing else. This connotes that if he put much on the table and there will be enough, they may give (to the host's children).

R. Yonah (Berachos 42b DH Bein): A guest may not give from his bread to the servant while the host holds his cup, lest the host object and the wine will spill. However, he may give to other guests. Since the host invited them, he is not particular about what they give to each other.

Poskim

Shulchan Aruch (OC 169:1): Any food brought in front of a person that has a smell, and people desire it, one must give from it to a waiter immediately. It is Midas Chasidus to give to him immediately from every kind (of food).

Mishnah Berurah (1): The same applies to anything sour.

Magen Avraham (1): One must give to him a small amount. If one stipulated at the time he hired him that he will feed him like other members of the household, he need not give to him from every food, only from those with smell. This is why we do not give to our servants from every food, for it is as if we stipulated. It seems to me that a stipulation does not help, for the law is due to pain. It seems that the law applies only to a waiter who serves, but not to a servant who sits at the table.

Kaf ha'Chayim (3,4): The SHLaH says that a stipulation exempts from Midas Chasidus for foods without smell. It does not help for foods with smell. The Magen Avraham says that it does not help at all. The SHLaH and Magen Avraham argue like the Rambam and Rashi. We need not give immediately to one who sits at the table, but it is improper not to give to him at all.

Mishnah Berurah (3): We learn from the Gemara that if one smells a food and desires it and cannot obtain it, he should spit out his saliva, for swallowing it can be dangerous.

Mishnah Berurah (4): It is Midas Chasidus to give from each food. Even if it has no smell and there is no danger, it pains him to see others eat and he cannot.

Shulchan Aruch (ibid.): One does not give to him as long as he or the host is holding the cup.

Rema: This refers only to the servant. One may give to someone else at the meal even in this case.

Beis Yosef (DH v'Chasav): It is forbidden only to give to the host's servant, child or slave. Guests may give to each other. Since the host invited them, he is not particular about what they give to each other.

Beis Yosef (DH u'Mihu): Rashi explains that one may not give to a waiter while the host is holding his cup, lest he get angry or worried that not enough will remain for the hosts, and he will choke or spill the cup. One may not give while the waiter is holding the cup, lest he spill it. R. Yerucham explains that one may not give while he (or the host) is holding the cup, i.e. the guest, lest the host look intently, and he will spill it.

Magen Avraham (4): R. Yonah (and the Beis Yosef, who cites him) connote(s) that one may not give to someone else who was not invited.

Mishnah Berurah (6): Some explain that one may not give while he himself is holding the cup, lest he sense that the host is looking on crossly, and he will spill it.

Mishnah Berurah (7): Some say that even the host should not give to the waiter while the waiter is holding the cup, lest he spill it and it will be disgraceful for those seated at the meal.

Kaf ha'Chayim (8): The Rema permits in the last case, when the host is holding the cup. One may not give to anyone holding a cup, lest he (be distracted taking the food and) spill it. However, according to R. Yerucham who says that we are concerned lest the guest spill because he senses that the host is looking on crossly, one may give to another guest even if the guest is holding a cup.

Bi'ur Halachah (DH l'Shamesh): One must give even to one who only cooks, and does not serve at the meal. The Gemara connotes that one must give to anyone who is there and smells the food. Rav Huna and the Shulchan Aruch discussed the waiter, for this is a typical case. This is why the custom is that when someone comes to the house during a meal, we offer him to eat.

Shulchan Aruch (170:19): Guests may not give the food in front of them to the child or slave of the Ba'al ha'Bayis, unless they received permission.

Magen Avraham (23): The Yam Shel Shlomo (7:20) connotes that it is forbidden to give only to the host's slave, but one may give to a servant serving at the meal.

Mishnah Berurah (41): Some permit giving to a servant. Some forbid.

Mishnah Berurah (40): The Magen Avraham permits if there is much on the table. All the more so it is permitted after they finished eating and there is food left on the table. Shemen Roke'ach leans to be stringent about this.

Kaf ha'Chayim (65): Shirei Keneses ha'Gedolah brings from Dameshek Eliezer that one may give from his portion. It is forbidden only to give his entire portion. Eliyahu Rabah says that the words of the Shulchan Aruch and Rambam connote unlike this. Eshel Avraham (23) permits giving a little only when there is much food on the table.

Kaf ha'Chayim (66): The Yam Shel Shlomo (20) says that we forbid giving to the host's child or slave, and all the more so to others. However, guests may give to each other.

Kaf ha'Chayim (67): A guest may give a small piece of a fruit, or from his own food. It is proper to do so, for this increases camaraderie.

Maharit (1:150 DH v'Od): The Gemara forbids giving to the son or daughter of the host, lest he be embarrassed. All the more so one may not give to others! Do not say that he objects only to his children, for they are like himself, but he is happy if they give to others. This is wrong. It causes him shame. It is considered theft. A Tosefta (Bava Kama 11:4) permits a son or slave eating at the table of his father or master to give a piece to the son or slave of the father's (or master's) close friend, for this is the custom of Ba'alei Batim. This implies that we permit only a son or slave. Alternatively, embarrassment applies only when one takes off the table, but not to a son eating with his father. Tosfos (Chulin 6b DH Eshes) says (unlike Rashi) that a woman may give some of her husband's food to a friend who is helping, for normally he would not mind. I proved that a guest may not give his portion to others. If one gave his portion to a man to be Mekadesh his daughter, even if you will say that the host is not upset, it is only Safek Kidushin. However, a guest may one of the fruits in front of him to someone else eating at the meal, for it is normal for guests to share their portions.