The Hupa ceremony begins with the Rabbi reciting the Beracha of “Boreh Peri Ha’gefen”’ over a cup of wine, followed by the Birkat Erusin. He then gives the cup to the groom to sip, and then the bride is given the cup to sip. Must the Rabbi also sip some of the wine, or does it suffice for the bride and groom to sip?
This question was addressed by the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in one of his responsa (Pe’er Ha’dor). He writes that the Rabbi must, in fact, take a sip of the wine, and if he does not, then his Beracha is considered a Beracha Le’batala (a Beracha recited in vain). The reason, the Rambam explains, is that the recitation of Birkat Erusin constitutes a Minhag (accepted custom), and not an outright obligation. This Beracha thus differs from Kiddush and Habdala, which are also Berachot recited over wine. The person who recites Kiddush and Habdala may have somebody else drink without drinking himself, since these Berachot are required according to Halacha. When the Beracha is obligatory, one can recite the Beracha of “Ha’gefen” and the obligatory Beracha for somebody else, who may then drink the wine. Indeed, Hacham Baruch Ben-Haim zt”l would give the Habdala cup to somebody else to drink after he recited Habdala. But when the Beracha is not obligatory, the person who recites the Beracha with “Ha’gefen” must drink the wine.
It is thus customary for the Rabbi to take a sip of the wine over which he recites the Birkat Erusin. In order not to drink directly from the cup that he will then give to the bride and groom, the Rabbi generally spills a few drops onto his finger and then sips it that way, thereby fulfilling the Halacha without drinking directly from the cup.
This Halacha does not apply to the second cup of wine used at a Hupa, over which the seven Berachot are recited at the end of the ceremony. The one who recites “Ha’gefen” over this cup does not have to sip from the wine, since these Berachot are, indeed, obligatory, and it is therefore sufficient for the bride and groom to drink some of the wine.
Summary: After the Rabbi recites Birkat Erusin at a wedding, he sips a bit of the wine and then gives the cup to the bride and groom. Regarding the second cup at the Hupa, over which the seven Berachot are recited, only the bride and groom need to drink from the cup.