What is the ruling on the prayer of someone who prayed Maghrib alone, then intended to make up previous prayers after joining another congregation?

Your prayer is valid, and you will receive the reward of congregational prayer for the prayer that was being made up. As for the timely prayer performed individually, you will receive the reward of one who prays alone. You should have interrupted your prayer and joined the congregational prayer that was being established, as this is a permissible excuse for interrupting prayer.