What is the ruling on a second congregational prayer during the Imam's sermon for Salatul-Istisqa (prayer for rain), and what should they have done?

If a group misses the Salatul-Istisqa (prayer for rain) but catches the sermon, they have the choice between sitting and listening to the sermon and then performing the prayer individually – because it has become a supererogatory (nafila) prayer – or leaving it. Some scholars hold that if Salatul-Istisqa is missed, it is missed. However, if they arrive while the Imam is delivering the sermon, they should pray two rak'ahs as Tahiyyatul-Masjid (greeting of the mosque) and then sit to listen to the sermon. As for forming a second congregation during the Imam's sermon, there is no evidence for its permissibility.