Others Drinking from the Kos shel Bracha
Regular Cup
When a person drinks from a regular cup and some of the drink is left over, he should not give the cup to his friend to drink. Perhaps the friend does not want to drink because he is worried there is some saliva there and the first person might be sick, putting him in danger too. By giving him the cup, one compels him to drink, as he is too bashful to refuse (שו''ע סי' ק''ע סט''ז ומ''ב סקל''ז ). However, there are some solutions, as will be explained.
Place it on the table. If one merely leaves it on the table rather than putting it in his friend’s hand and the friend takes it on his own, there is no problem, as he is not compelling him to do anything. The friend willingly drinks assuming the baal habayis is not sick (מ''ב שם ).
Wiping the spot of the mouth. If the first drinker wipes the spot where he put his mouth on the cup, he may present the cup to others, as wiping that spot makes it not repulsive (שו''ע סי' ק''ע סכ''ב ).
Family. Many people allow family members to drink from the cup of the baal habayis because the family members know the baal habayis is healthy and free of illness (בן איש חי שנה א' בהר אות י''ג ).
Kos shel Bracha
The widespread minhag is to drink from the kos shel bracha, e.g., from the kos used for Kiddush or zimun, as that is the best way to do the mitzva (שו''ע סי' ק''צ ס''ד ). People do this even if they are not family without worrying that it is repulsive or potentially dangerous because it is for a mitzva purpose (שדי חמד אסיפת דינים פאת השלחן ברכות סי' ל''א ד''ה ואולי ). Still, it is proper to wipe the rim of the cup where the mouth was placed in case this concern exists for a kos shel bracha too (ע''פ קו' הגרש ''ז אוירבאך, שש''כ פמ''ח הע' ס''ט ).
