Is the prayer valid for someone who pronounced the opening takbir while the Imam was rising from prostration and before he had stood up straight?

It is sunnah for the congregant to join the Imam in whatever state he finds him. So, he performs the opening takbir and then prostrates with him if he finds him prostrating. If the Imam rises from prostration before the congregant prostrates, the congregant should rise and not prostrate.

If the congregant finds the Imam rising from prostration, he should perform the takbirat al-ihram but not descend to prostrate, because he has missed the rukn (pillar), and he should wait for the Imam while standing.

As for the opening du'a (du'a al-istiftah), it is not recited in this situation according to the majority of scholars, because what the congregant catches is considered the end of his prayer. This is also the view of the Shafi'is, because the congregant did not find the Imam in the standing position.