halacha - Can you join a minyan in another room?

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?