Is Friday Prayer obligatory for mosque guards, and is their speech or return of greeting during the sermon considered idle talk that invalidates the Friday Prayer?

If guarding the mosque is necessary to protect it or to protect the worshippers during Friday prayer, then the guard is permitted to miss Friday prayer and pray Dhuhr instead. He is considered excused, and he is allowed to speak and return greetings. However, if guarding is not necessary during prayer time, he is not allowed to miss Friday prayer. If the guard attends Friday prayer, he must listen to the sermon like everyone else, and he is not allowed to speak to return greetings except by gesture. However, if the speech is for an important purpose, there is no harm in it.