What should I do if I join the 'Isha' prayer in congregation from the first rak'ah, but I haven't prayed the Maghrib prayer yet?

If someone misses Maghrib prayer until the Isha prayer is called, they have two options: They may join the congregation with the intention of praying Maghrib. When the third rak'ah is completed, they should wait until the Imam sits after the fourth rak'ah, recites the tashahhud, and makes the salutation. Then, they should pray Isha. Alternatively, they may pray Isha with the congregation, and then make up the Maghrib prayer only, without repeating Isha.