אחר שסיים ברכת המזון מברך בורא פרי הגפן ויטעום המברך [ואם הפסיק בין הברכ' לטעימה ע"ל סי' רע"א סעיף ט"ו] ואח"כ יטעמו האחרי' אם כול' זקוקי' לכוס אחד ונותן המברך מכוסו לכוס ריקן שבידם לא יטעמו עד שיטעום הוא אבל אם אינם זקוקים לכוסו יכולים לטעום קודם שיטעום הוא. אין צריך המברך לשפוך מכוסו לכוס המסובין אלא אם הכוס המסובין פגום (ועיין לעיל סימן קפ"ב וס"ס רע"א):

After concluding the Birkat HaMazon, one blesses "Creator of the Fruit of the Vine", then the blesser tastes [from the cup of wine]. After that tasting, others [at the meal] taste [from the cup of wine] if all of them are connected to one cup, and the blesser gives from their cup to empty cups in their hands. They should not taste from the cup of blessing until the blesser tastes. But, if they are not connected to that cup, they are able to taste prior to the blesser tasting. The blesser need not spill from their cup to the cups of their fellow diners, unless the cups of the fellow diners are unfit.

After one drinks the cup of Birkat HaMazon, one blesses the abbreviation of the three blessings.

There is a lack of agreement as to the amount of wine-drinking that obligates one in needing to say the concluding blessing, whether it’s sufficient with an olive's-volume or with a quarter-log. Therefore, one should be careful to drink either less than an olive’s-volume or a quarter-log in order to remove oneself from doubt. And, here, it's not possible to drink less than an olive’s-volume, since everything that requires a cup requires drinking from it like a cheeksful, which is a majority of a quarter-log. Therefore, one should drink an full quarter-log.

If the blesser does not want to taste, one of the fellow diners should taste [at least] the minimum amount [of wine], and two people's drinking does not combine to count for the minimum amount. And, nevertheless, it is a preferable performance that all of them should taste [from the cup of blessing].

When the diners are at a large meal, and they do not know whither the Birkat HaMazon cup reaches, each and every of those in doubt needs to bless Creator of the Fruit of the Vine if it arrives to them.