Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem (vol. 4, p. 345), discusses the conditions that must be met for the Sheba Berachot to be recited at a meal for a bride and groom during the week after the wedding. One condition is that the meal must have been held specifically in the couple’s honor. The source of this ruling is the Sefer Hamichtam (Rabbenu David of Narvona, 13th century) in Masechet Sukka (25), who writes that the phrase "She’ha’simha Bi’m’ono" is not recited before Birkat Ha’mazon if a bride and groom are present at a Berit Mila celebration. Even though they are present, this phrase is omitted because the meal was not held in their honor. This would certainly apply to the recitation of the seven special Berachot, as well. This is also the ruling of the Meiri (Rabbi Menachem Meiri, 1249-1310), who writes that it is "obvious" that the seven Berachot are not recited at a meal that is not held in the couple’s honor, such as if they attend a meal to give honor to the host. The work Zichreh Kehuna, citing the Meiri’s ruling, likewise adopts this position.
Interestingly enough, however, Rav Haim Palachi (Turkey, 1788-1869), in his work Ru’ah Haim (Eben Ha’ezer 62:11), addresses the case of a groom who, during the week after his wedding, attended with his friends the Berit Mila celebration made by his brother in a different home. Regarding such a case, Rav Haim Palachi wrote that the Sheba Berachot should be recited, since the groom is in attendance. However, Rav Moshe Halevi clarifies that Rav Haim Palachi issued this ruling because in the case under discussion the meal was made both in honor of the Berit, and in honor of the groom. It is only for this reason that the Sheba Berachot should be recited. But if a groom attends a meal that was not made in his honor, the blessings are not recited.
It occasionally happens that a bride and groom will attend Se’uda Shelishit in the synagogue on the Shabbat after their wedding. The meal was not specifically made in their honor, but they happen to be praying Minha at the synagogue, and they decide to join the congregation for Se’uda Shelishit. In such a case, the Sheba Berachot are not recited, even though both the bride and the groom were present and partook of the meal, since the meal was not made specifically for them. If the synagogue formally dedicated the meal in their honor, such as if it added special foods or invited special guests in honor of the Simha, then the blessings would be recited. But if the congregation is conducting an ordinary Se’uda Shelishit meal and the bride and groom happen to attend, the Sheba Berachot are not recited.
Summary: The Sheba Berachot are recited only at meals specifically held in honor of the bride and groom. If the bride and groom happen to be present at a meal that was not made in their honor, the Berachot are not recited.
