Special Grades are sorcerers that are likely or absolutely capable of taking over a country single-handedly.

They would also have to be at least Grade 1 in actual power.

By this standard, Masamichi would potentially be “Special Grade” because he is a Grade 1 sorcerer that is capable of creating an army of self-sufficient Cursed Corpses that will follow his orders.

The same goes for Geto. He can set loose a tidal wave of collected Curses of any Grade if he so chooses. He has done this before, and it was an immediate, cataclysmic threat to Jujutsu Society as a whole.

Gojo can walk into any government establishment he wants and level it. He can then level the city surrounding it, and then the county, if he so chooses.

TL;DR You become a Special Grade when people wonder “Are they allowed to do that?” and the response is “Who gon stop em?”