The Prajñā Group (般若衆, Hannyashū) is a regiment of the ANBU of the Land of Woods. They were all but annihilated by Root, but at the time of the Kage Summit, there were still remnants of the group out for revenge. The seventeen who tried to assassinate Danzō Shimura were killed by him. In the manga, they fought with extremely long kunai that lacked rings at the end.
Prajñā is a Buddhist concept, often translated as wisdom. There are many interpretations of Prajñā, but it usually refers to the wisdom that is able to extinguish afflictions and bring about enlightenment.
In the anime, the sword-length kunai used by these ANBU were replaced with regular kunai and swords.
Prajñā is also a translation of the word "hannya", which is a mask used in noh theater to represent a jealous female demon. This is in reference to the design of the mask being worn by all the members.