By Rabbi Dovid Ostroff
The laws and customs of bentching with wine – Siman 182-183
When should one bentch with wine or grape juice?
We find various opinions amongst the Rishonim as to whether one should bentch with a ëåñ.
Tosefos learns that whenever one bentches, one must bentch with a ëåñ and recite áåøà ôøé äâôï at the end of bentching. According to this opinion, if two people dined together, each must have his own ëåñ, just as each person bentches on his own. If three people dined together, one person can recite Birkas Hamazon and be îåöéà the other two diners and one ëåñ will suffice. 
Midrash Ruth cites that one is required to bentch with a ëåñ when three dine together. A single diner is not required to bentch with a ëåñ.
The Rif and Rambam learn that even three are not obligated to bentch with a ëåñ.
The Vilna Gaon writes  that all opinions agree that if one has wine (or grape juice) in one’s home than one should bentch with a ëåñ.
How does the Shulchan Aruch pasken?
The Shulchen Aruch cites these three opinions and does not direct us to a p’sak. The Bach and Maharshal rule that one is obligated to bentch with a ëåñ but the custom is to follow the third opinion and not bentch with a ëåñ.
The Rama writes  that nevertheless (even though one is not obligated to bentch with a ëåñ), it is a îöåä îï äîåáçø  to do so. Consequently the Mishna Berura writes that when one has wine or grape juice, it is a îöåä îï äîåáçø to do so when three people or more dine together. 
Rav Benzion Abba Shaul ztz”l  also writes that one should try to bentch with a ëåñ when three adults dine together.
It seems that most people do not bentch with a ëåñ during the week, only on Shabbos. 
When should everyone refrain from speaking?
The person reciting the zimun should not talk once handed the cup of wine to say zimun. The other diners must not speak once the mezamein begins zimun until he drinks from the ëåñ, unlike those who talk once they have concluded their own Birkas Hamazon.
There are opinions that learn that the other diners follow the example of the mezamein, and from the point he may not speak, they may not either. The Mishna Berura writes that it is right to follow that opi;nion. 
Who drinks from the kos?
Obviously the mezamein drinks, and he drinks first.  He then passes the wine to his wife to drink, because a wife is blessed from her husband’s ëåñ ùì áøëä. It is a îöåä îï äîåáçø for all diners to sip from the ëåñ. 
When is it a mitzvah for other diners to drink from a ëåñ?
The Shulchan Aruch writes  with regards to drinking wine Friday night that it is a îöåä îï äîåáçø for all participants to sip from the ëåñ ÷éãåù. We see that the Mishna Berura wrote that all diners should also sip from the ëåñ, which would seemingly apply to the ëåñ of Sheva Berachos as well, and yet many refrain from doing so. Is there a source for that?
Rav Shlomo Zalman Auerbach ztz”l points out  that the Shulchan Aruch rules  that after one drinks from a cup one should not hand it to someone else to drink from, lest that person would rather avoid drinking from someone else’s cup. Because he is handed the cup he is embarrassed to decline even though we see that it is a mitzvah to drink after the mekadeish or mezamein. He says that perhaps this is the halachic source why people, other than family, do not usually drink from the ëåñ.
How much of the kos should the mezamein drink?
Halachically it is sufficient to drink the majority of a Revi’is  but then one encounters the problem of a b’racha ahcharona, because there are opinions that hold that one recites al hagefen after drinking a k’zayis, so the Shulchan Aruch says  that one should imbibe the entire revi’is and subsequently recite al hagefen.
 Siman 182 and M”B 3.
 Cited by M”B siman 182:4.
 Siman 182:1.
 Preferred and special merit.
 M”B siman 182:4.
 àì”ö ôî”å ë’.
 HaGaon Rav Sternbuch shlita.
 Siman 183:22.
 Siman 183:4.
 M”B siman 183:19.
 Siman 271:14.
 See SS”K48 footnote 69 and the úé÷åðéí åîéìåàéí.
 Siman 170:16.
 Because the day Kiddush is only áåøà ôøé äâôï and if one does not drink, one has not had Kiddush.
 See SS”K 50:9 and footnotes 17-18.
 More than 44cc.
 Siman 190:3.