Parashas Eikev · Passuk 8:10
When you have eaten your fill, give thanks to the ETERNAL your God for the good land given to you.
Torah Temimah points to Yerushalmi Berakhot / Rosh Hashanah 29a theme.
TT lemma: ואכלת ושבעת וברכת (unit 7 of 14).
Sefaria — Yerushalmi Berakhot / Rosh Hashanah 29a theme
English: It was taught: For all mitzvot, one who is exempt may still discharge the obligation of the many — except Birkat HaMazon, as it is written “you shall eat and be satisfied and bless”: the one who ate is the one who must bless. The intent is: one who has already blessed for himself cannot discharge others who ate, because only one who ate and is blessing for himself can include others in that blessing.
Custom (who may be motzi others in Birkat HaMazon)
Unlike shofar or megillah, one who has already said Birkat HaMazon cannot discharge those who have not yet eaten and blessed.