The Bei'ur Halacha (supplement to the Mishna Berura commentary to the Shulhan Aruch by Rabbi Yisrael Kagan, Lithuania, 1839-1933), in Siman 170, decries the practice of guests who arrive late at weddings or other receptions. He observed that occasionally guests arrive towards the end of the meal, when the host no longer has what to serve them, thus causing him embarrassment. Guests invited to an affair must show sensitivity to the host and ensure to arrive on time so that he can host them properly.
It is forbidden for a wedding guest to take home an extra Birkat Ha'mazon card or Kippot for his friends of family without receiving permission from the host. Similarly, a wedding guest may not take some food home with him from the affair for his children. Since the host provided the food to be eaten at the celebration, one is not entitled to bring home food without the host's permission, or even if it can be assumed that the host doesn’t mind.
If a waiter brings an extra portion of food as a friendly gesture to a guest in the restaurant, it is forbidden for the guest to partake of that portion, unless the restaurant's owner had authorized the gift. Since the waiter is merely an employee, and does not own the food in the restaurant, he is not legally entitled to distribute the food in a manner that has not been sanctioned by the restaurant's owner. Eating the extra portion of food may therefore constitute Gezel (theft). Similarly, if a bus driver invites a passenger to ride free of charge, the passenger must insist on paying. Here, too, the driver works as an employee and does not own the bus; he is therefore not authorized to offer free rides without the knowledge of the company's owner.
(Taken from Dayan Shelomo Cohen's work "Pure Money," vol. 1, p. 21)
Summary: Guests invited to a reception must ensure not to arrive so late that the host may not have any food to feed them. Guests at a wedding or similar function may not take extra Birkat Ha'mazon cards, Kippot, or food home from the affair without the host's permission. A restaurant guest must refuse offers of free portions of food made by a waiter, unless the waiter had been authorized by the restaurant's owner to make such an offer.
