When Sheba Berachot are recited after Birkat Ha’mazon, two cups of wine are poured. One is held by the Mezamen – the one who leads Birkat Ha’mazon – who holds the cup while reciting the Zimun and throughout Birkat Ha’mazon, as usual. There is then a second cup which is brought around right after Birkat Ha’mazon to different people who recite the special blessings of the Sheba Berachot while holding the cup. After the recitation of all the special blessings, the Mezamen recites the Beracha of "Gefen" over his cup of wine.
The question arises as to whether the Mezamen is required to hold the cup throughout the recitation of the Sheba Berachot. May he put down the cup after he concludes Birkat Ha’mazon, and then lift it again when the time comes for him to recite "Boreh Peri Ha’gefen," or must he continue holding the cup during the recitation of the Sheba Berachot?
Hacham David Yosef (contemporary), in his work Halacha Berura, rules that the cup should be held throughout the recitation of the Sheba Berachot. He draws proof from the comments of Tosafot to the Gemara’s discussion in Masechet Berachot (51) concerning the proper way to hold the cup while reciting Birkat Ha’mazon. The Gemara addresses the question of "Semol Mahu She’tesayei’a Be’yamin" – whether one must hold the cup only with his right hand, or if the left hand can also be used to support the cup. Tosafot comment that this question was asked regarding unique circumstances when holding the cup with only one hand during Birkat Ha’mazon could be difficult, and they give three examples: 1) when using a very large cup; 2) on Rosh Hodesh Tebet which falls on Shabbat, when Birkat Ha’mazon is very lengthy (due to the additions of "Al Ha’nissim," "Reseh" and "Ya’aleh Ve’yabo"); 3) at a wedding, where Birkat Ha’mazon includes Sheba Berachot. The implication is that the Mezamen must hold the cup throughout the recitation of Sheba Berachot, and thus the question was posed whether one may use both hands, given the difficulty entailed.
By contrast, Rav Efrayim Greenblatt (1932-2014), in his work Ribebot Efrayim, ruled that the Mezamen is not required to hold the cup throughout the recitation of the Sheba Berachot. He writes that Tosafot might have been referring to a case where the Mezamen himself recited all seven Berachot (as opposed to the common practice of giving others the honor of reciting the blessings), and for this reason, he is required to hold the cup throughout the entire recitation. Tosafot’s comments thus do not prove that the Mezamen must continue holding his cup of wine while other people recite the Sheba Berachot.
In light of the different opinions, it would seem that the Mezamen should continue holding the cup if he can do so without much difficulty, but if he finds this hard, he can put it down, in accordance with the lenient position of the Ribebot Efrayim.
Alternatively, the Mezamen may rely on the position of the Hacham Sevi (Rav Sevi Ashkenazi, 1656-1718), who ruled that if the Mezamen has a medical condition that makes his hands unsteady, such that he cannot hold the cup of wine, somebody else may hold the cup for him during Birkat Ha’mazon. In light of this ruling, a Mezamen at a wedding who finds it difficult to hold the cup throughout the recitation of Sheba Berachot may have somebody else hold the cup for him when he feels he needs to rest his hand.
In conclusion, it should be noted that this discussion underscores the great value and importance of the Kos Shel Beracha – reciting Birkat Ha’mazon over a cup of wine, and holding it throughout Birkat Ha’mazon. The very fact that the Poskim found it necessary to discuss whether the cup may be put down during the Sheba Berachot shows us that this is a very valuable and precious Misva which should be observed whenever possible.
Summary: The person who leads Birkat Ha’mazon at a wedding or Sheba Berachot should continue holding the cup of wine after Birkat Ha’mazon, throughout the recitation of the Sheba Berachot, though if he finds this difficult, he may put it down, or, preferably, give it to somebody else to hold when he needs to rest his hand.