It is very common for the Rabbi or somebody else to recite Habdala on Mosa’eh Shabbat on behalf of all the people in the synagogue. The people listen attentively to the recitation of Habdala and answer "Amen," thereby fulfilling their obligation. Often, immediately after answering "Amen" to the final Beracha, the people exclaim "Shabu’a Tob" and extend greetings to one another, even before the person who recited Habdala had a chance to drink the wine. The question arises as to whether they fulfill their obligation in this fashion, or if perhaps they cannot fulfill their obligation since they interrupted in between the individual’s Beracha and his drinking of the wine.
The Shuhan Aruch discusses a similar question amidst his presentation of the laws of Kiddush (Orah Haim 271:15; listen to audio recording for precise citation). He writes that if a person speaks after he recites Kiddush but before drinking the wine, he must repeat the Beracha of "Boreh Peri Hagefen." Since he must drink the wine to fulfill the Misva of Kiddush, and his Beracha over the wine was disqualified by his speaking, he must repeat the Beracha over the wine. However, the Shulhan Aruch adds, a person in this case does not repeat the Beracha of Kiddush. Since he recited Kiddush properly over a cup of wine, he has fulfilled his Kiddush obligation despite speaking before drinking the wine.
This demonstrates that speech in between the recitation and the drinking affects only the Beracha over the wine; it has no effect at all on the Kiddush.
We can now return to the situation of the people who announce "Shabu’a Tob" after Habdala, before the person who recited Habdala drinks the wine. As we saw, interruptions between the Beracha and the drinking affect only the Beracha over drinking – "Boreh Peri Hagefen." The congregation, who fulfill their obligation by listening to the one reciting Habdala, are not required to drink the wine, and thus interrupting in between the Beracha and the drinking has no effect upon them. Once they have listened to the entire Habdala recitation over a cup of wine, they have fulfilled their obligation, even if they speak before the person who recited Habdala drinks the wine.
Of course, if that individual speaks before he drinks the wine, then he must repeat the Beracha of "Boreh Peri Hagefen" over the wine – just as the Shulhan Aruch ruled regarding Kiddush. However, his interruption has no impact upon the congregation, as they are not required to drink the wine or wait for him to drink the wine.
Summary: If one person recites Habdala on behalf of the congregation, they fulfill their obligation through his recitation even if they speak immediately after he completes Habdala, before he drinks the wine. If the individual who recites Habdala speaks before drinking the wine, he must repeat the Beracha of "Boreh Peri Hagefen" and then drink the wine, but this does not affect the congregation.
