How many people amongst the Minyan must eat bread in order to be able to recite Sheva Brachos?
Sheva Brachos may only be recited after a meal in which a Chasan and Kallah are present if there is a Minyan of ten men above the age of Mitzvos in the room. Of these ten men, a certain amount of them must have eaten bread during the meal in order to be allowed to recite Sheva Brachos. However, the Talmud, Rishonim, and early Poskim do not express how many bread washers are needed during the meal, and we hence find a debate amongst the Poskim in this regard. The following are the opinions in this matter:
The opinions: Some Poskim entertain that perhaps ten men must wash on bread during the meal in order to be allowed to recite Sheva Brachos. Other Poskim, however, rule that the criteria necessary to say Sheva Brachos is similar to that of saying a Zimun with Elokeinu, and hence it suffices to have seven men present who washed on bread, and another three who ate other foods to an amount that requires an after blessing, in order to say the Sheva Brachos. Other Poskim imply that it suffices to have six men who washed on bread. Other Poskim rule it suffices to have three people who washed on bread and another three who ate another food, and the other four do not have to eat at all. Other Poskim rule that it suffices to have three people who washed on bread, and the remainder of the Minyan do not even have to eat anything at all.
The final ruling: Practically, the custom is like the second opinion above, to follow the same laws as a Zimun with Elokeinu, and hence of the ten men, seven are to wash on bread and another three eat a Kezayis or Revius of another food. All ten men have to be prior to saying their after blessing at the time of Birchas Hamazon. If one went ahead and recited their after blessing prior to Birchas Hamazon, then just as they no longer join for the Zimun of ten so too they no longer join for the Sheva Brachos. Nonetheless, it is not necessary that all ten recite the after blessing at the same time, and some may do so after the communal Birchas Hamazon.
The laws of Sheva Brachos follow the same criteria as a Zimun of Elokeinu and hence of the ten men, seven are required to wash on bread and another three to eat at least a Kezayis or Revius of another food.
Must the Panim Chadashos eat at the meal?
There is no requirement for the Panim Chadashos to eat during the meal of Sheva Brachos, and hence so long as there are seven men who washed on bread, and three who ate other foods that require an after blessing, the Sheva Brachos may be recited [even if the Panim Chadashos simply stands nearby]. [However, the guest has to be someone honorable enough to the Baalei Hasimcha that they are joyed by his presence and would give him food if he wanted.]
Must the Chasan and Kallah wash on bread?
The Chasan and Kallah are both to wash on bread during the meal in order to allow the Sheva Brachos to be recited.
May one who did not eat at the meal recite Sheva Brachos?
Some Poskim rule it is not necessary for the person saying the blessing to have eaten anything by the meal. Other Poskim rule that only those who ate bread during the meal may say the Sheva Brachos. Practically, the widespread custom is to [only] allow those who ate something during the meal to say Sheva Brachos, even if they did not eat bread.
 See Pischeiy Teshuvah E.H. 62:8; Otzer Haposkim 17:18; Yabia Omer 3:11; Nitei Gavriel Nissuin chapter 102
 Michaber E.H. 62:4 “Birchas Chasanim [i.e. Sheva Brachos] may not be recited after Birchas Hamazon unless ten adult men are present”
 For the fact that Sheva Brachos may only be said after a meal over bread: See Michaber ibid who writes “Birchas Hamazon”; China Vichisda Kesubos 1 p. 112b; Shoel Vinishal 3:419 based on Tosafus Pesachim 102b; Otzer Haposkim 17:34; Shraga Hameir 1:58; Halacha Lemoshe 5; Nitei Gavriel ibid 102:1
 Pischeiy Teshuvah ibid in name of Zechor Leavraham
 Birchas Habayis 62:19; Shoel Umeishiv Telisa 1:198; Bireich Es Avraham 71:22; Leaning opinion of Pischeiy Teshuvah ibid in name of Zechor Leavraham that majority suffices
 Regarding the necessity to have seven men who washed and three who ate a Kezayios/Revius of other foods in order to recite Elokeinu by a Zimun-See: Admur 197:2; https://shulchanaruchharav.com/halacha/how-many-people-must-eat-to-make-a-zimun-of-ten-with-elokeinu/
The reason: As the Sages did not enact the saying of Sheva brachos unless a Zimun is recited with Hashem’s name.
 Leaning opinion of Pischeiy Teshuvah ibid in name of Zechor Leavraham that majority suffices; Pnei Yitzchak 1 Mareches Brachos 1 Kuf; See Yabia Omer 3:11 and Otzer Haposkim ibid who write they in truth intended to say 7 and not 6.
 Minchas Pitim 62; Poskim recorded in Otzer Hapoksim ibid; Nitei Gavriel ibid footnote 6
 Halef Lecha Shlomo O.C .93; Poskim recorded in Otzer Hapoksim ibid; Nitei Gavriel ibid footnote 7; See Pischeiy Teshuvah ibid in name of Zechor Leavraham who seems to question that perhaps not even a Zimun is required, and so long as ten people are in the room it suffcies.
 Yabia Omer ibid; Sova Semachos 1:26; Nitei Gavriel ibid
 Nitei Gavriel 102:3; So rule regarding a Zimun: Admur 197:4; M”A 197:4; Elya Raba 197:6; M”B 197:9
 Nitei Gavriel 102:4; Sova Semachos p. 116; See Nesuin Kehilchasan p. 433 who questions this matter and as to whether all seven must recite Birchas Hamazon together
 Opinion in Rama 62:7; Ran Kesubos
 Chelkas Mechokeik 62:11
 Chelkas Mechokeik 62:11
 Halef Lecha Shlomo O.C .93; Tzitz Eliezer 13:99; Yabia Omer 6:9; See Kisvei Daas Kedoshim p. 84; Hanesuin Kehilchasa p. 532; Nitei Gavriel 102:5
Other opinions: Some write that a Chasan and Kallah are not required to wash on bread in order to recite Sheva Brachos. [Sova Semachos 1]
 See Nitei Gavriel 102:6
 Zechor Leavraham 2:2 questions this matter; Yaskil Avdi 8:20-28; Beir Moshe 2:118; Shraga Hameir 1:58; Rivivos Efraim 4:227
 The reason: So is proven from the fact that the Panim Chadashos is not required to eat.
 China Vechisda Kesubos 1 112b; Pnei Yitzchak 1 Mareches Brachos 1 Kuf; Cheshev Haeifod 1:9; 2:3; Yabia Omer 3:11-7
 Nitei Gavriel ibid; See Beir Moshe ibid