Wed Sotah 44: we'll explain Sotah 38

R' yehoshua b. Levi says that an iron wall cannot separate one to his father in heaven.

Tosfos bring another Gemara that applies this statement to a Minyan, that someone can join even behind a wall.

Tosfos asks: that we see that one cannot combine to a Minyan if they’re' behind a wall?

Tosfos answers: that's only if there is no 10 people in one place, and he wants to combine to make a Minyan. Or, if the Shatz is in one place and wants to be Moitzie the Minyan in a second place. However, if the Minyan and Shatz is in one place, a person in the next room can be Yoitza from the Shatz. After all the gemara in Rosh Hashana says that the Shatz is Moitzie the people out in the field.