Q. If a chassan is in shul, the tzibbur skips tachanun. Question: one who came late and is davening privately, is he also exempt? How late could one come to shul and still be considered exempt due to the chassan’s presence? For example, one who started SE when they were already up to Aleinu – would he be exempt?

A. Horav Shlomo Miller’s Shlit’a opinion is that as long as the chosson’s minyan did not end, even at aleinu, if he reaches tachanun then, he is exempt from saying it.

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