In order for ten men to form a Minyan, they must all be situated in the same room. In some circumstances, though, two rooms might perhaps be considered a single room. One such situation is where a narrow room opens into a wider room. If some men are in the narrow room, and some are in the wider room, can they form a Minyan?
The basic principle in such a situation is that a minority of the people can be seen as joining the majority if the majority is in the large room, but not in the reverse case. Meaning, if nine men are together in the wider space, and one man is in the narrower space, they can form a Minyan, because we view the lone individual in the smaller space as if he is together with the other nine in the larger area. In the converse case, however, they cannot form a Minyan. Meaning, if nine men are in the smaller area, and one is in the larger area, we cannot view the lone individual in the larger area as being together with the other nine. If five people are in the larger room and five in the smaller room, then in this case, too, they cannot form a Minyan.
If the Hazzan is in the narrow room, and everyone else is in the wider room, then we may view them as being together, such that they form a Minyan.
If there is a Minyan in the smaller space, and some people are in the larger space, the Hazzan must be a person situated in the smaller space. No one in the larger area can serve as Hazzan.