לזכות חנה ליבא בת נעכא גיטל
There is a common question whether a person can be considered davening bi-tzibbur when he is standing outside of shul or in the Ezras Nashim [when there are no women present of course]. The are actually two questions - 1] If there are only nine in one room - can a person in a different room be considered the tenth? 2] If there are already 10 and he just wants to be considered as davening bi-tzibbur.
Here is an answer to the second question.
The truth is that the Ran in Rosh Hashana says that if he is standing outside and answers with the tzibbur הרי הוא כעומד בפנים - it is like he is standing inside [cited in Halacha Brura Vol. 4 page 180].
The Mishna Brura discusses this question - and rules that מעיקר הדין it counts as long as he and the tzibbur can see each other. In Rav Soloveitchik's time the Mishna Brura didn't have the authority it has today. Today, any Rav worth his salt knows the Mishna Brura - not so "back in the day" so it is not surprising that he didn't mention the Mishna Bruras psak. I am just surprised that Rav Shachter [who wrote the sefer] didn't mention the Mishna Brura.
See Piskei Tshuvos [עמ' תצו-תצז] that it seems that we can be even more lenient than the Mishna Brura.