Can you join a minyan in another room?

Question

I recently davened (prayed) at a hassidic Rebbe's minyan. I found something peculiar, namely that the Rebbe didn't actually daven in the same room as everyone else. Instead, he davened in a different room with the door closed at all times. To indicate that he had completed shmoneh esrei there was a knock or sound that came from his room. After asking I was told that he always davens like this and other hassidic Rebbes do as well. I would like to know what the halachic basis is for this practice and are there any other sects of Judaism that do things similarly?

Answer 1

See Shulchan Aruch Horav (OC 55:22, 591:14) where he says that if there is 10 people in the room then anyone, in another room/house, who hears them could be Yotse with them.