While the Ma'mum stands to make up for the missed Rak`ahs, the Imam performs Sujud-ul-Sahw
Q 1: I was offering Zuhr (Noon) Prayer in congregation. I joined them in the third Rak‘ah (unit of Prayer). When the Imam (the one leading the congregational Prayer) finished the Salah (Prayer), he offered Taslim (salutation of peace ending the Prayer), so
I stood up to complete the parts of the Salah I had missed. However, when I stood upright performing the parts I missed, the Imam prostrated to perform Sujud-ul-Sahw (Prostration of Forgetfulness).
What should I do in such situation?
If you stood to complete what you had missed from the Salah and had already stood fully upright, then the Imam prostrated to perform Sujud-ul-Sahw, you should continue with performing the parts of Salah you had missed and not sit to perform Sujud-ul-Sahw with the Imam. This is because, if you starts a Rak‘ah, it is not permissible to go back (in the steps). However, you have to perform Sujud-ul-Sahw after completing the parts you have missed, and it is better to perform it before the Taslim (salutation of peace ending the Prayer).