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 Berachos / Rosh Hashanah 29a theme.
TT lemma: ואכלת ושבעת וברכת (unit 7 of 14).
Sefaria — Yerushalmi Berachos / Rosh Hashanah 29a theme
English: It was taught: One who is exempt from a commandment can discharge others’ obligation, except for Birkat HaMazon, as it is written, “you shall eat and be satisfied and bless” — the one who ate should bless.
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.