The gemara in Ketubot (7b) describes the seven berakhot recited under the chupa during the nisuin process. The gemara claims that these berakhot are recited only on the first day of the chatan's seven-day yom tov. During the remaining seven days, all seven berakhot are recited only if there are panim chadashot ('new faces') present; if no new individuals attend, only the final berakha is recited. How does the presence of panim chadashot extend the seven berakhot to all seven days?
A simple answer might be that if new people attend the subsequent meals, the seven berakhot must be repeated for THEIR benefit. Part of a person's obligation to rejoice the chatan and kalla involves the recitation of these berakhot (many of which are direct blessings wished upon the chatan and kallah), and these new arrivals have yet to fulfill this mitzva.
This view is contradicted by an interesting Tosafot (Ketubot 1b s.v. vehu) which lodges two interesting statements about panim chadashot. Tosafot claim that only individuals who add to the simcha environment recreate a chiyuv to repeat the seven berakhot. Presumably, friends, relatives or generally respected individuals would fall into the category of those whose presence augments the simcha. If the repetition of these berakhot were intended to allow the newcomers the opportunity to fulfill their mitzva, we should recite the berakhot whenever anyone new arrives. Evidently, Tosafot felt that panim chadashot revitalizes the simcha and creates a new obligation to recite the berakhot. Even the people who have already recited the berakhot previously are obligated to repeat them when experiencing the higher simcha generated by the arrival of panim chadashot. However, the revitalization of simcha can occur only if valued or significant individuals make their first appearance during the week of simcha.
A second indication that Tosafot view panim chadashot as stimulating new simcha is their position that on Shabbat the seven berakhot are recited even without panim chadashot. Since Shabbat obligates special meals and simcha, the effects of panim chadashot are attained even without the arrival of new people. Had panim chadashot meant simply that the new arrivals must recite the berakhot, we would not regard the experience of Shabbat as a substitute.
An important issue stemming from the definition of panim chadashot might be the question of whom we consider "new." If someone attended the original wedding seuda but did not attend the berakhot (neither the berakhot recited under the chupa nor those recited after the wedding meal), would his attendance during the ensuing week (for potentially his first round of berakhot) trigger an obligation of new berakhot? If panim chadashot obligates newcomers to recite berakhot, anyone who has not yet recited or answered berakhot would fall into this category. If, however, panim chadashot – by their first appearance at the celebratory events - increase the general mood of simcha, someone who already celebrated the wedding (even if he did not hear berakhot) might not qualify as panim chadashot. This issue - of how we define the term 'chadash' - is the subject of an important machloket Rishonim (see the Ritva for an elaboration of the different shitot).
Another debate among the Rishonim surrounds a situation in which the panim chadashot departed before the berakhot were recited. For example, panim chadashot attended the actual meal during the week but left before birkat ha-mazon. Some claim that on a day in which panim chadashot arrived the seven berakhot may be recited the entire day. Other Rishonim, though, claim that berakhot are recited only in the physical presence of panim chadashot. Conceivably, if panim chadashot generate new simcha and hence a renewed obligation of berakhot, the obligation might outlast their presence. Even after they leave, the new level of simcha might be sustained, thus compelling a new set of berakhot. If, however, the berakhot are their obligation, yet to have been fulfilled, we would not recite berakhot in their absence.
Would individuals (such as ketanim) who do not comprise a minyan of ten necessary for the recitation of sheva berakhot, constitute panim chadashot? If there are ten adult men, but all of them have already participated in sheva berakhot, would the arrival of newcomers who are ketanim allow or obligate new berakhot? Again, if the arrival of panim chadashot launches a new level of simcha, these ketanim (assuming their presence adds to the simcha) perhaps inspire a higher degree of simcha for the adults which obligates them to repeat sheva berakhot. Obviously, if the panim chadashot themselves must fulfill the mitzva, we cannot define ketanim as panim chadashot. The Pitchei Teshuva (Even Ha'ezer 62:14) cites a discussion concerning this issue.