Many people sitting in the Musjid do not join the Janaazah Salaat.

Answer

Those who do not join in the Janaazah Salaat are not sinful. Janaazah Salaat is Fardh-e-Kifaayah. Nevertheless, they are depriving themselves of much thawaab. But no one has the right of criticizing them for abstaining from Janaazah Salaat. Furthermore, it is not permissible for Hanafis to perform Janaazah Salaat inside the Musjid.