Answering Kedushah from outside the room

Hi,

One who is outside of a room in which a minyan is davening should answer Amen and recite the responses to Kedushah. However, one who has already davened, and is in a space from which he can hear many minyanim, is not obligated to remain there in order to respond to all of them.

(Rivivot Ephraim 1:89:1)

Have a great day,
Mordechai