Is it permissible for a man and a woman to pray in congregation together, and how should they stand?

There is no objection to a woman following a man in congregational prayer, even if he is a stranger to her, as long as no forbidden seclusion (khalwa) results from it. The prescribed manner is for the woman to stand behind him, based on the hadith of Anas (may Allah be pleased with him): "So I and the orphan stood behind him, and the old woman stood behind us." If she stands beside him, her prayer is valid, though it is contrary to the Sunnah. This is forbidden if it leads to seclusion, due to the Prophet (peace be upon him) saying: "No man should be in seclusion with a woman unless there is a mahram with her."