The Ge’onim (Babylonian scholars in the post-Talmudic era) were of the opinion that when a person recites Kidush on Shabat, he does not fulfill the obligation unless he personally drinks the minimum required shiur (quantity) of wine from the Kidush cup. Even if someone else at the table heard the Kidush and then drank the shiur, nobody fulfills the obligation if the mekadesh (person who recited Kidush) did not drink the minimum required amount.
The Shulchan Aruch, however, does not follow this opinion. Although it is preferable for the mekadesh to drink the required amount, if this is not possible, then he may give the cup to someone else to drink. Assuming that the person heard the Kidush and the mekadesh had intended for his kidush recitation to fulfill the obligation for this person, that person may drink the shiur of wine, and then everyone who heard Kidush fulfills their obligation.
How can a group fulfill the obligation of Kidush if no one is able to drink the minimum required amount of wine?
The Shulchan Aruch cites an opinion that a group can fulfill the obligation even if the shiur is drunk collectively, meaning, if several people each drink a small amount, and together they drink the minimum required amount.
According to the accepted halacha, however, this opinion may be relied upon only Be’di’avad (after the fact), meaning, if the mekadesh recited kidush and no one drank a complete shiur. In such a case, they do not need to repeat kidush, and may instead rely on the view that the collective consumption of the shiur suffices to fulfill the Mitzvah.
However, this may not be planned from the onset. What should a group of people do if they know from the start that no one in the group is able to drink a complete shiur of wine?
Rav Moshe Ha’Levi (a”h), in his sefer Menuchat Ahava, states that in such a case, the group should recite kidush on bread and not on wine. This is halachically preferable to everyone drinking only a small sip of wine.