Q. If you are late and only arrived (to a flaishig siyum on the nine days) after the siyum was done, can you still join and take part in the seudah?

A. 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 seudoh, you can join them.

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