Q. It says we cannot daven mincha in front of a Tzibur that was Mekabel Shabbos. (accepted Shabbos). Can one do melacha in front of them such as a person off the street coming in to put on a light in a shul?

A. Indeed if one is late for Mincha, he should daven privately by a secluded corner or in another room.
However, if one is walking in the street on Shabbos to attend his later Minyan and is asked by the people of the shul who received Shabbos earlier, and it is also obvious that he is just now going to his shul, he may help the early people and light for them.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a