Q. If someone was invited to a yohrtzait siyum during the Nine Days but arrived after the siyum, can he still partake, eat meat and drink wine?
Does he have to eat bread or at least mezonot?

A. On question 196 in regards to people who didn’t learn joining a siyum and eating meat during the Nine Days we wrote; “Remoh (O.H. 551: 10) rules that when someone finishes a tractate of the Talmud and makes a siyum, he can eat meat and drink wine on the Nine Days and the ones that belong to the banquet may join him, even though they did not participate in the learning, Mishna Berura (ibid. 75) explains that those are the people, related to him or his friends, that would have come to the siyum, on any other occasion and not only during the nine days. (see also; Talmud Shabbos 118b). Some Poiskim require an actual invitation, (Oilas Shmuel 56 – Hisorerus Teshuva 1:104 and Mekadesh Yisroel, Bein Hametzorim 136).
Horav Shlomo Miller’s Shlit’a opinion coincides with the above, however he does not require one to actually be invited.”
On question 199 we wrote; “Horav Shlomo Miller Shlit’a ruled that even if you arrived after the siyum, if you also finished the Masseches or are from the ones that belong to the siyum (see prior question), even if you came late and were not present at the end reading of the Talmud, if the people are still at the seudah, you can join them.”
Piskei Teshuvos (551: 38: n. 204, quoting Poskim), also maintains that one can partake in the siyum, eat meat and drink wine, even when he arrived late and is not eating bread or mezonos.
Horav Shlomo Miller’s Shlit’a opinion is similar.

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