In Birkat HaMazon, the one who is saying Birkat HaMazon answers amen to his own bracha during the third bracha, Boneh Yerushalayim, as required by the text.
Why is it that here we say amen to our own bracha? If it is because it is the end of a "group of brachot" why would we not say amen to our own brachot in other instances, for example the 19th bracha in Shmoneh Esrei?
If someone is listening to the one saying Birkat HaMazon, when would her or she answer amen to the bracha of Boneh Yerushalayim; would it be before or after the one saying it says his own amen?