Q. If a chassan joins the seudah shlishit on Shabbat, without it being planned out before, do you still say sheva brachoth?

A. Meiri (Kesuvos 7b) writes that no sheva brochos are recited if the meal was not prepared specifically in the choson-kalo honor. A similar opinion is found in Orchois Chaim (61) in regards to a bride and groom participating in a Bris-Mila feast and in Imrei Dovid (41) on Seuda Shlishis.

Ohalei Yeshurun (4: 5) quotes Horav Moishe Feinstein Zt”l advising that when sheva brochos is done in a restaurant where people eat all the time, you should announce that this particular meal is a dedicated one and the bride and groom should sit in a prominent seat to make this known.

Some Poiskim maintain that during an ordinary seuda shilshis, if you add a new dish or serve a previously unavailable food, you sing wedding nigunim and say words in the honor of the choson-kalo, you can recite sheva brochos. (Minchas Shlomo 3: 103 – Teshuvos Vehanhogos 1: 749 – Hanissuin K’chilchoson 14: 23). Horav Shlomo Miller Shlit”a is of the same opinion.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a