I heard that when anyone else is listening to someone make kiddush or havdalah, l’chatchilah they must be either sitting or standing depending on what the person making kiddush or havdalah is doing. In addition, the Chazon Ish indicates that if a person did not do so he/ she is not yotzei even b’di’eved. Can you please tell me the source for this?
SA OC 396:6
The source from the Hamotze Bracha is in the Beit Yosef ,Orach Haim [קסז][י-יא].he also brings some source from the Gemara [ברכות מב ב-מג ג] .The idea is that you show that you are a group and you all are yotzei in one Bracha,by all sitting/standing together.But,from there it is easy to conclude that if the mevareich explicitly has chavana for you,you are yotzei,standing or sitting.
(I dont have enouth rep' to comeent this + I dont have any books here.can someone check and edit?)
also in Mishna Brura [רעא מו],we can see something aout this ,that the sitting(that his Psak) is for kviut and not only a part of the Kidush.