halacha - How much of benching should be said aloud in a zimmun?

When a zimmun (rabbotai nevarech, etc.) is done, I have heard that the first three paragraphs should be said aloud by the leader.

(That'd be:

Baruch Ata Adoshem Hazan Et Haolam......Baruch Ata Adoshem Hazan Et Hakol

Nodeh Lecha Adoshem.. bchol ayt uhvechol shahah

VeAl Hakol....Baruch Ata Adoshem Al Haaretz VeAl Hamazon.

)

I am wondering if just the end of the first and third have to be said aloud? I wonder this as the first and third end in actual brachot.

What is the rule for what has to be said aloud?

I notice there are other brachot after that though (e.g. UhVenai yerushalayim....Baruch ata adoshem bonai verachamav yerushalayim amayn) and I haven't seen that said aloud, whereas with the harachamans some do say them aloud.

What is the rule?

Are there differences between ashkenazim and sephardim in this regard? Apparently, sephardim say a bit more aloud like harachamans, but I'd like confirmation of this and preferably a source.

Answer

When there is a Zimmun, what is supposed to happen is the leader recites the entire Bentching aloud on the group's behalf and everyone else listens with the intent to fulfill their obligation and replies Amen at the relevant places.

If one is unsure of his ability to pay attention to the leader the whole time (this is common), he should recite in an undertone along with the leader but be careful to get a few words ahead of the leader at points where an Amen should be said in order that the listener can still respond Amen to the leader. (Shulchan Aruch OC 183:7)

Some (Magen Avraham ad loc. and endorsed by Mishna Berura and Aruch haShulchan) hold that for Ashkenazim the above trick (reciting along in an undertone) only works beginning after haZan et haKol because for Ashkenazim until that point is still considered the essential part of Zimmun (cf. Rama OC 200). (For the curious, Friday night Kiddush and the first blessing of Bentching have almost the exact same number of words to focus through.)