What is the ruling on the prayer of one who led a prayer while menstruating, and did not inform some of the congregants to repeat the prayer because it was difficult to reach them?

If the blood is istihadah (non-menstrual vaginal bleeding), then ablution is obligatory after the time for prayer begins. If someone prayed in ignorance of the Imam's impurity, their prayer is valid. The prayer of the congregants is valid and they are not required to repeat it. The Imam is obligated to perform sincere repentance.