Should one pray Maghrib or Isha if the Maghrib prayer was missed and the Isha congregational prayer was caught?

The preponderant opinion is that a difference in intention between the Imam and the follower does not affect the validity of the prayer. So, if you missed the Maghrib prayer and found people praying 'Isha, it is permissible for you to join them with the intention of praying Maghrib. If you catch the first raka'ah with them, then at the completion of your prayer in the third raka'ah, you should sit and not continue with the Imam into the fourth raka'ah. You can either intend to separate from the Imam and make the salutation, or you can wait for him until he sits for the tashahhud (definition) and make the salutation with his salutation, and this is better.