It is permissible to start the prayer while sitting with the imam in the second row, and it is permissible for those in the first row to start the prayer while standing with the imam.

Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) narrates from the Messenger of Allah (peace and blessings be upon him) regarding the prayer of fear that the Messenger of Allah (peace and blessings be upon him) stood up while one group stood behind the Messenger of Allah (peace and blessings be upon him) and the other group was sitting behind the standing group. All their faces were towards the Messenger of Allah (peace and blessings be upon him). The Messenger of Allah (peace and blessings be upon him) said the takbir, so both groups also said the takbir and began the prayer. Then when he (peace and blessings be upon him) bowed (for ruku‘), the group standing behind him also bowed, while the other group remained sitting. Then when he (peace and blessings be upon him) performed the prostrations, they also performed the prostrations, while the other group remained sitting. Then when he (peace and blessings be upon him) stood up, they also stood up and moved back until they sat in the place of their companions. The other group (came forward), so the Messenger of Allah (peace and blessings be upon him) led them in one rak‘ah with two prostrations, while the other group remained sitting. Then the Messenger of Allah (peace and blessings be upon him) said the salam after the tashahhud. Then both groups stood up and each performed one rak‘ah with two prostrations for themselves (and after tashahhud, said the salam).