Assallamualaikum wa rahmatullahi wa barakatuhu...If someone forgets to read a surah after al-fatihah (as in asr or zhuhr) in the first is that salah regarded as invalid for both the imam and the people praying behind him?
If a surah or the wajib amount prescribed by shariah is not read after fatiha in the first rakat by accident 'sajdah sahw' must be done for the salah to be valid. If 'sajdah sahw' is not done the salah will become invalid and it will have to be repeated. This applies to both imams and people praying individually.
As muqtadis do not pray surahs behind the imam there salah is dependent on the validation of the imam’s salah. والله أعلم