When three or more men ate together and recite Birkat Ha’mazon with the introductory Zimun, it is proper for the one leading the Zimun to hold a cup of wine during Birkat Ha’mazon and then drink the wine after Birkat Ha’mazon.
The Shulhan Aruch (Orah Haim 183) discusses the various laws and customs that apply to the cup of wine over which Birkat Ha’mazon is cited. After the leader recites the introductory verse of "Abarecha Et Hashem Be’chol Et," before he recites, "Vayedaber Elai Zeh Ha’shulhan," somebody else should lift the cup with both hands, and give it to the leader, who should receive it with both hands and then transfer it to his right hand. After this point, his left hand should not even touch the cup. The leader should then lift the cup such that the wine is at least one Tefah (approximately three inches) from the table, and he should hold the cup in that position until he recites the Beracha of "Gefen" and drinks a revi’it (about 3oz) of wine after Birkat Ha’mazon. If this is too difficult, he should lift the cup one Tefah off the table at least through the end of the first Beracha of Birkat Ha’mazon.
After Birkat Ha’mazon, the leader should give the cup to his wife so she can drink some of the wine. Drinking the wine of this cup brings a woman a blessing for fertility, as well as other blessings, and so even if the woman is already beyond childbearing age, she should drink some of the wine.
If one’s wife is a Nidda, he should not give her the cup to drink, but if other people will also be taking some of the wine before the cup reaches the wife, then the husband may send it to his wife.
Summary: When Birkat Ha’mazon is recited over a cup of wine, the leader of the Zimun should be given the cup before reciting, "Vayedaber Elai Zeh Ha’shulhan." Somebody else should lift the cup at this point with both hands, and the leader should receive the cup with both hands and then transfer it to his right hand. He should hold the cup one Tefah (three inches) off the table until after Birkat Ha’mazon. After Birkat Ha’mazon, the leader’s wife should be given the cup to drink some of the wine.
