After reciting Kiddush, the Mekadesh (person who recites Kiddush) should drink at least a "Melo Lugmav" – or approximately 1.7 ounces – of wine from the cup. If he cannot drink, then he may give the cup to somebody else to drink. Once this quantity of wine has been drunk from the cup, everyone who heard Kiddush has fulfilled their obligation, even if nobody else drank.
Nevertheless, the Shulhan Aruch rules that there is a "Misva Min Ha’mubhar" – an added Misva, beyond the strict obligation – for everyone to take a sip from the Kiddush cup. Therefore, many people have the custom to pass the Kiddush around the table after Kiddush so everyone can take a sip, in fulfillment of this special Misva. (Our custom is to pass the cup around in age order.) It goes without saying that if a person does not wish to drink, he is not required to, and he has fulfilled his obligation of Kiddush despite not drinking any wine, as long as he heard Kiddush and somebody drank the minimum required amount.
The question arises, however, as to whether it is indeed proper for everyone to drink from the same cup. The Shulhan Aruch (Orah Haim, end of 271) writes that one should not drink from a cup that is "Pagum" (literally, "blemished"), meaning, a cup from which somebody else has drunk. Once a person puts his lips onto the cup and drinks, it is considered inappropriate for another person to then drink from that cup. Seemingly, then, it would be improper to pass the Kiddush cup around the table for people to drink after the Mekadesh has drunk from the wine. How, then, do we fulfill the Misva Min Ha’mubhar to drink from the Kiddush cup?
Several answers have been given to this question. The Hafetz Haim (Rav Yisrael Meir Kagan of Radin, 1839-1933), in Sha’ar Ha’siyun (271), answers, very simply, that the rule of "Pagum" does not apply to the "Kos Shel Beracha" – the cup upon which a Beracha was recited. This cup is special, and it is therefore a Misva to drink from it even after somebody had drunk from it previously. The Hafetz Haim makes this point also in Mishna Berura (182) in regard to the cup over which one recites Birkat Ha’mazon. That cup, too, is customarily passed around after the one who led the Zimun drinks from it. The Mishna Berura writes that as this cup is a "Kos Shel Beracha," it is not subject to the law of "Pagum."
Rav Haim Palagi (Turkey, 1788-1869), however, proposes a solution to avoid this problem. He writes that once one adds more wine to the cup, it is no longer considered "Pagum." Therefore, he advises that the Mekadesh should pour wine from the bottle into the cup, and then pour the wine into other cups for the people around the table. Rav Palagi adds that the Mekadesh should preferably have in mind that his Beracha should cover all the wine on the table, and not merely the wine in the Kiddush cup. This way, all the wine on the table is considered the Kiddush wine, and so even if the original Kiddush wine is finished before everyone drinks from it, they can take wine from the bottle on the table to fulfill this Misva.
Another option, which is indeed customary in many homes, is for everyone at the table to have a cup of wine in front of them during the recitation of Kiddush. Since the Mekadesh presumably has this wine in mind as he recites the Beracha, all these cups have the status of "Kos Shel Beracha," and the people at the table can fulfill the Misva by drinking their cups.
Unfortunately, many people make the mistake of pouring from the Kiddush cup into other cups after drinking from it. As we have seen, the people at the table can drink from the Kiddush cup even though others had drunk from it, but if the wine is "Pagum" and then poured into another cup, drinking from that cup does not fulfill the Misva. Therefore, one of the aforementioned solutions should be followed, namely, everyone should drink from the original Kiddush cup, wine should be added to the Kiddush cup after the Mekadesh drinks, or everyone should have a cup of wine in front of them for Kiddush.
Hacham Bension Abba Shaul (Israel, 1923-1998) addresses the question of whether a cup becomes "Pagum" if somebody drank from it using a straw. He concludes that one should not drink from the cup in such a case, unless the person sucked wine into the straw and then took the straw out of the wine before drinking it. Hacham Bension also writes that if somebody drank directly from a bottle, the bottle is considered "Pagum" even if his mouth did not touch the bottle and he poured the water out of the bottle into his mouth.
Summary: Although it is not obligatory for everyone to drink from the Kiddush wine, there is a special Misva to do so. Generally, one should not drink from a cup after somebody else had drunk from it, but nevertheless, the Kiddush cup may be passed around the table so everybody can drink from it. Some require adding more wine to the cup after the Mekadesh drinks from it, and then distributing the wine. If everybody has a cup of wine in front of them during Kiddush, they may drink that wine instead of drinking from the Kiddush cup. One should not, however, pour from the Kiddush cup into other cups for the others to drink from, unless he first adds wine to the Kiddush cup.
