Ashk'nazi prayer books generally include the line
ברוך הוא וברוך שמו
after the zimun (invitation to say birkas hamazon and before the actual birkas hamazon. It generally (in my experience) is preceded by the instruction:
יחיד אינו אומר
yachid eno omer, which means "an individual does not say" or "someone alone does not say"; the word yachid is often used to refer to someone not part of a minyan.
Who should say this, and who should not — and why?
Ideas that come to mind are:
- Anyone included in a zimun should say it, but not someone saying birkas hamazon without a zimun.
- Anyone saying birkas hamazon with another says it, but not someone alone.
- Anyone included in a zimun of a minyan says it, but not someone saying birkas hamazon without a minyan.
- Anyone leading a zimun (the m'zamen) says it, but no one else.
I'd appreciate any argument or, especially, source. Naturally, for practical guidance, each person should rely on his rabbi rather than answers here.