Some Shafi'i brothers recite a verse during Fajr prayer that requires Sajdah and go straight into prostration and then stand up again to continue the Surah. Is this allowed in Shia Islam? If I am praying in congregation with them how should I act?
It is not allowed during obligatory Prayer (Salah) to add Rokoo' or Sojood because it will invalidate the Salah. Reciting any of the four verses of obligatory Sajda is not allowed in obligatory Salah. If you do Sajda for reciting or listening to any of the four verses of obligatory Sajda, you Salah becomes invalid and you need to perform the Salah again.
Wassalam.