If the prayer is ongoing you should pray isha with the congragation and then pray your magreb. If the prayer is not ongoing you should pray magreb. If you missed magreb prayer for a reason that is not legitimate, you should not make it up as it wouldn’t be accepted.

Please can you enlighten me on how to observe missed salat given the fact that there is another congregational prayer going on. For example if i miss salatul magrib and by the time i want to observe this salat ,salatul ishai is already been said. Now should i join them and pray slatul ishai after which i pray my salatul magreb, or should i pray salatul magreb thereby missing the congregational praying of salatul ishai?