What is the ruling on a man leading an unrelated woman or his maharim in prayer, and where should each of them stand in relation to the male imam?

A woman stands behind a man alone in a row, whether she is his mahram or a non-related woman. However, it is not permissible for a man to pray alone with a non-related woman if no one else is with them, due to the forbidden seclusion (khalwah) involved. But if there are others present, there is no objection. This is indicated by the hadith of Anas in the Two Sahihs: "So I and the orphan stood behind him, and the old woman stood behind us."