A person who joins the Imam before rising from the ruku'. He

A person who joins the Imam before rising from the ruku' has gotten that raka'ah. In congregational prayer, if a person joins the Imam while he is in the ruku' (bowing position), then he should say the takbir and perform the ruku' with the Imam. After that, he should say "Sami'allahu liman hamidah" and stand up to join the Imam in the next raka'ah. In this case, the person has joined the prayer with the Imam before rising from the ruku' and he has gotten that raka'ah. However, if the person joins the Imam after the Imam has risen from the ruku', then he has missed that raka'ah and needs to complete it after the Imam has finished the prayer. The person should not join the Imam at all if the Imam has already said the taslim (salam) to end the prayer. And there is no such thing as Sujudul Badi in Islamic prayer.